P.S. re: falling asleep in the movie - getting out at all is a win. Being comfortable enough to fall asleep is actually quite a healthy sign IMHO. Have you given yourself permission to have fun yet? Do you feel comfortable smiling and laughing? It is possible to love deeply, grieve strongly, and still enjoy good things in your life. Grief and happiness are weirdly compatible once you allow them to share the same space.
Put a time limit on your expeditions. “I’ll go to this event for 20 minutes.” If you’re enjoying yourself, stay longer.
Go to a nearby bus stop. Talk to a stranger there “What bus are you waiting for? Do you know when it gets here?” Take the bus for a few stops and talk to a passenger.
Get some food. Go to a nearby park and eat outside. Talk to any children there.
I want to validate your feelings while also telling you you’re wrong. 
I totally get questioning yourself because things haven’t worked out. But this problem is incredibly common in our society, and it sounds like you’ve gotten where you are at least partly through self-sacrifice (moving for family) and bad luck (marriage ending). It also sounds like you’ve been taking reasonable steps to address the problem (doing stuff on your own, volunteering) but for various reasons those things just haven’t panned out (bad luck again.) I don’t know you, of course, but I haven’t seen anything in your posts that makes me think you’re the problem. You aren’t trash-talking your soon-to-be ex or your daughter for having her own life. You aren’t even venting at the faceless idiots who made the freaking parking situation so freaking complicated. You seem honest and introspective and willing to try. All things that point to you not being the author of your current misery, but also reason to believe you’ll be able to scramble out of this hole eventually.
I really relate to the blood drive story. I was in Key Club in high school and I always felt like our volunteer activities were so pointless. One time we handed out cups of water at the finish line of a 5k fun run. The cups had already been filled and placed on a table, and the runners were coming in at such a slow trickle they could have easily grabbed their own. Another time we went to an animal shelter and walked the dogs, which was great I guess, but there were more volunteers that day than dogs that needed walking.
It’s taken me a while to find my place, or places, in the world of volunteering. A lot of it is about matching your skills and interests to an organization that can put them to good use. It’s harder than finding a job, honestly. I’m a lawyer, so I do some law stuff in my spare time. A local immigrant rights organization periodically puts on Naturalization Fairs for green card holders looking for affordable assistance in becoming citizens; I’m on their email list and help out when I can. I also do Name and Gender Change Clinics for trans folks through a different organization, and Criminal Record Expungement Clinics through yet another organization. It has taken me a while to find all these groups, and my schedule doesn’t always line up with theirs (WHY do so many legal aid organizations think I and other lawyers can leave work on a weekday in time to get downtown by 5pm?!?) but it’s incredibly rewarding work that reminds me why I went to law school and, in my darker moments, why I’m worthy of all the oxygen I suck up. I also volunteer as a patient escort at an abortion clinic; that took YEARS to get into. I spent forever trying to get on Planned Parenthood’s radar before happening to meet someone who volunteered with a different group and got me in. Then I had to go through training, but after that it was easy to sign up as often as I wanted. Some friends of mine volunteer at an animal shelter, but they had to wait six months for the orientation. I’m thinking of volunteering my scuba diving skills at the aquarium, but I need more experience and a higher level of certification, then I need to go to one of the semiannual orientations, then I need to commit to nearly every weekend for a year, so that might not happen for a while. I recommend thinking about what interests you and then asking around about how you can get into that, and being prepared for a bit a slog at first. But the payoff is totally worth it.
NWYE, I relate to your experience of going to an event and then turning around and leaving.
I find myself avoiding things in expectation of being in that situation. For me though, it’s not usually a feeling of being overwhelmed, but simply annoyed at all the bother. I want to go to a baseball game, but it’s such a PITA with the parking, the traffic, dealing with asshole companies like TicketMaster… I just can’t be bothered. My job has me out in the world doing complex things, so when I’m not working I just want it all to be easy. This keeps me from going out much.
I’m trying to use Uber to simplify the travel annoyances. I try to remind myself when I get bored and antsy at home that I could be out somewhere. My compromise is usually to eat lunch at a local place I like, and sometimes that leads to sociability at the bar, even though I don’t drink.
Not sure how helpful this is to you, except to say, I get it.
You guys are all so great and I can’t thank you enough for taking the time to give such thoughtful responses.
I wish I could hug each of you.
Another thing I do to motivate myself to get out and do things is think “I better get out and do things while I’m able to. The best days are the mundane ones where nothing in particular is wrong and you’re free and able to do what you want.”
Also you’re more likely to regret not doing things than doing them. Sometimes adventures work out, sometimes not but you gotta try.