I'm sorry. I know how much it sucks to be left out of things. My brother and I (as adults) were both left out of my dad's wedding a few years ago. Other family members were part of the wedding party and virtually everyone in the family had some role (even things like decorating, etc), except for us, despite our numerous offers to help. We weren't even included in a single picture taken at the wedding. We tried expressing how important it was to us to be included in some (any!) way before the wedding, but it only seemed to fall on deaf ears, so I can understand why you don't want to bring anything up to your friend. I also understand how it doesn't really mean the same if someone includes you after you ask rather than them doing it from the very beginning. For us, even trying to discuss with our dad after the wedding has only led to more bitterness.
My best advice is to allow yourself some time to go through whatever emotions you have (don't just bottle them up inside), then find a way to let it go as much as you can. I KNOW it's not easy, but find some sort of release because dwelling on things only exacerbates the hurt.
I also understand the emotions that accompany feeling left out of things due to a disability. I'm not in a wheelchair, but do have physical issues that restrict my range of motion and overall mobility. It sucks knowing there are some things you just physically CAN'T do. I think sometimes friends/family members assume I can't do something and/or don't want to hurt me by inviting me to something that I can't do, so they just leave me out of certain activities.
Might I suggest finding some things you CAN do that make you happy/make you feel good about yourself and when things open up in the world again, invite friends and family to do those things with you. For instance, I can no longer go ice skating or play hockey like I used to, but a couple years ago I invited my brother and his girlfriend out to a hockey game and we had a blast. He told me afterwards, "I don't know why I haven't thought of us going to a game together before, but let's do it more often." It was a way for us to enjoy hockey, though in a different way, but still together.