You're such a good mom for trying to help her. Anxiety is a weird thing. I developed it during a very bad marriage. I deal with it now by working with a therapist and learning triggers (clutter really bothers me if I'm around it for a long time, its a strange, just low level thing, but it's also incredibly easy to just not have a ton of crap everywhere). Sometimes I know I just need to "call it." Holidays are the worst for me. People have crap all over their houses, and it's so much obligation; I really hate the whole season, so sometimes, I just don't. Your daughter's triggers could be anything but figuring them out will definitely help her! Good luck! It can get better.
Clutter really bothers me too as do family get-togethers for various events. I handle them, but they exhaust me.
This was me the first panic attack I had. I get them at night sometimes, when I'm asleep and the first time I woke myself up having one was so scary even though I knew by that point, what it was.
I hate the panic attacks that happen while I'm sleeping, & I wake up w/ my heart racing & feeling like it's going to pound out of my chest!
I'm also going to throw this out there, too. My DD20 just wrote a paper on research-based negative effects of social media on college age people. It was super interesting. She talked about how depression and anxiety rates in college age people are five times higher than they were in same age people during the depression era. The gist of it was this: social media highlights social status and popularity, and those who feel they don't measure up can feel subordinated and envious of others, which can lead to depression and anxiety; envy being the key to these feelings. Looking at others' postings and photos makes it seem like everyone else is out having a good time and someone only looking is not. Whereas posting photos themselves was found not to be associated with feelings of envy, because it gives them the sense that their lives are fun, too. Heavy social media use and having a high number of followers was also cited as causing more symptoms. Her conclusion was that "People need to realize that people post about the best parts of their lives and not the bad or negative parts, which can make others feel their lives aren't as good". I think it's important information for our young people today to realize, as their lives are consumed by social media. Things some of us can't imagine, like losing followers, or having a snap left open, can send some of these people into a complete tailspin. I think it's healthy for them to perhaps learn to put down their phones and to do things without them. All things to consider. (And OP I think it's also healthy for your DD to learn how to be strong and content with herself even if her boyfriend isn't around! That might be something she can talk to the therapist about.)
I think there is a lot of truth in this.
Every now & then, I find I have to essentially walk away from all social media for a while. The news stories & links, the different postings from FB friends, the events... it all combines to make me feel stressed & anxious. Even the birthday greetings - I start to worry that, while I wished one FB friend a "happy birthday," I missed wishing someone else a "happy birthday" & what if that person saw my "happy birthday" to the other person & is now wondering why I didn't wish him/her a "happy birthday"? And have I "liked" too many posts? Not enough posts? Is my not posting anything about the flag kneeling going to offend my military BIL who is posting all kinds of stuff? But, if I do post something, will that offend my friends who are in support of the NFL players?
It's maddening. And I feel sure a lot of people do NOT put this much thought & angst in FB & just go about their lives happily. But I overthink EVERYTHING, & Facebook just becomes another outlet for my overthinking.... so, every so often, I just have to walk away.
That is what came out at me in the OP. She's been in a relationship since she was 15 and has been having anxiety since he's been gone, even asking you questions about what she should do about going to work if her legs were weak, etc. Not criticizing, just saying it's what came out at me. (You asked!) I don't know what their relationship is like, but you've also said you are her "safety". I can appreciate that because my DD and I are very close, too. But I guess what I'm trying to say is that the timing of these episodes may not be just coincidental. If she's used to depending on her boyfriend (even if she doesn't realize it), or you, then she may be becoming a little anxious when faced with making decisions on her own. No? Maybe? Just someting to think about and perhaps work on with the therapist.
Earlier in this thread, I said my DH is my "safe space". I can't explain how. It's not that I can't make decisions on my own. It's not that, when he's not here, I can't function on my own. I'm reasonably intelligent. I can do things on my own.
But, in some intangible, indescribable way, he grounds me.
Family get-togethers make me anxious, but I've found if he's w/ me, I can handle them much better. If he is working on the day some kind of family get-together is scheduled & the kids & I go alone, I'm much more anxious.
A few years ago, we were gathering for my mom's birthday w/ my parents & my sister & her family, & I couldn't explain it, but I felt so panicky & anxious. I just didn't want to go. At all. And this was MY family - not even DH's. In the past, I would have called my mom & my sister both my best friends. But, had DH not been there, I think I would have had a serious anxiety attack right there in the restaurant. Later, I found out that my sister & her husband were in the middle of a pretty serious marriage crisis. And I don't know if I sensed it or what, but my anxiety was seriously ramped up.
And, while I used to be a lot closer emotionally to my mom & my sister, I think, the longer I've been w/ DH, the stronger my emotional connection to DH has become while I've emotionally drifted away from my mom & sister.
They are no longer "my persons" - DH is my person.
Anyway, OP, I think it's good that you are acknowledging your DD"s anxiety & trying to find ways to help.
I was an anxious, nervous, shy little girl. The night before family get-togethers, I would throw up. When all the family (grandparents, aunts & uncles, cousins) were eating together, I had to eat separately in a room by myself because I was so nervous.
But no one ever told me I was okay. No one ever acknowledged my anxiety. I basically had to muddle through & learn ways to cope on my own - or not cope but still had to do whatever or go wherever.
A few months ago, one of my FB friends posted about her little girl who also has anxiety & finding her the right kind of help. And I cried. Because I was so thankful this mother was helping her little girl & helping her to learn the tools she needs now as she's growing up. While my parents realized I was shy & nervous, I basically just went through it on my own. I'm not explaining it well, but I didn't felt seen... instead, I felt like it was just something I had to get over.
Fastfoward to now, & I feel guilty about my anxiety. I think a lot of people think people use anxiety as an excuse, so, if I say anything, I worry that people will just think I'm either making up some kind of excuse or calling attention to myself. And I was raised to not call attention to myself & to not make a scene or cause problems for other people.
there is a reason this is happening but I'm not sure it is because her boyfriend is gone. He is gone like this every year. She is very strong and makes decisions on her own without the need of help from anyone - that's for sure. In fact, she keeps toying with the thought of wanting to be a police officer. She's gone on several "ride a long" in our community. So she is definitely a tough nut!!
My own experience and others in my personal life have not been. Bad enough to tell others if there is way to avoid them then please do so. My GP prescribed them very easily and the effects were horrid and that was just a couple of doses. Tried a few different ones. Fortunately I was able to find a doc that set me with a different route to take. If that works for you that's great but in my opinion if there is way to avoid them that's better.
I was hoping just having the Xanax in the house would have the same affect on me, that's why I agreed to fill the prescription even though I wasn't keen on taking the pills, but it did just the opposite. Everytime I considered using it, my anxiety would spike even higher as I started to worry about how it would affect me, what if I had a bad reaction, what if it became a crutch? 
(God, I was such a mess back then before my hormones got under control.) In the end, those now-expired pills are still sitting untouched in a medicine cabinet somewhere.
Let it go! 
. My own DGM (who lived to 103) controlled her anxiety by taking to an extreme "a place for everything and everything in its place". My DM deal with her anxiety by cleaning/rearranging (we'd come home from school and if all the living room was rearranged, we knew she had had a REALLY bad day!). Now I deal with mine by absolutely minimizing clutter, whether it be physical (stuff), technological (facebook, the news, etc) or familial (love you,but can't handle spending time with you right now). My DH is also a grounding tool for me (although I try not to abuse it).
