I totally understand about being depressed when the trip did not go according to plan. I'm a big planner too, and sometimes that sets me up for disappoinment, because I just want everone to have fun and for the trip to be perfect (which is, unfortunately, impossible!)
My advice would be to start planning another vacation, even if its a ways away! Then, do what you can to minimize planning every detail, and just have a list of rides and/or restaurants that you're excited about. Decide that you will be sure to hit those at some point in your trip, but that its more about enjoying the sights and sounds and excitement of Disney. Try to only make reservations for those restaurants/experiences that are a MUST for the whole family (and spread the reservations out). Then do your best to hit each ride as you come to it. Don't rush from ride to ride and land to land trying to hit all the best rides in one day, just walk along and enjoy each land as you come to it. I found out that I'd missed out on a lot of exciting details on my first trip just trying to cover as much ground as I could. The next time, I took it slower and went land by land. If there's a big line, utilize the Fastpass system or decide if you want to come back later and stand in line. Also, using the rider switch option with little kids can be helpful. If possible, next time try to give your family an extra day in the parks. Having that cushion helps a lot with catching up on rides you may have missed out on in the first part of your trip.
Make sure to have snacks on hand so the kids don't get hungry and cranky, and try to stay as close as possible to the parks and/or utilize the ART so you can take a break in the middle of the day and come back in the evenings. Of course, even using all these tips doesn't guarantee that nothing will go wrong, but its important to remember the FUN parts.
Maybe make a scrapbook or a photo collage of the GOOD parts of your trip, it will help the tantrums and disappointments fade into the background. Our last trip was far from perfect, with my 3 year old complaining daily that he "wanted to go home" (after I'd spent the entire year planning for a FUN family time!) But the few pics I have of him smiling with Donald and having fun on Monster's Inc. help me remember that the WHOLE trip wasn't a bust!
Also, I would seriously consider going back for a short trip without the kids if that's something you could do or want to do. It's nice to be able to take your time and go to some of the attractions that aren't as kid-friendly. And watching the fireworks together in front of the castle is pretty magical!
I guarantee that when the kids are older, they will remember the fun parts of the trip. And if you can take them back later when they can appreciate it more, and you know better what to expect (busy times & rides) and what to avoid (
Legoland) - then you can have a much better trip next time!
One more big tip - try to hit the parks in early to mid-May for less crowds and beautiful weather. You might run into a few rehabs and less attraction options, but the short lines are well worth it!
Well, I guess that's about all the advice I have. I'm sorry your daughter was sick (that's one of those nasty unavoidable downfalls!) and I hope you can go back to DL again and appreciate all the fun it has to offer. Some of the best memories I have of our vacations have very little to do with activities I'd "planned". Seeing my son's face when he really recognized characters from movies he'd been watching, and hearing my daughter make a wish while tossing a penny in the water at It's a Small World are those little things that you notice (and remember later) when you're not overly stressed about sticking to a schedule, and you can just relax and take in the Happiest Place on Earth! Best wishes!