As a 40-something myself, also pretty much alone (by choice), I have had to think about this as well.
What you are looking for to feel good is respect. Are you acting in a way that is worthy of respect? Fact of the matter is, earning respect is hard. Yes, it can be done through career, but it can also be done in how you treat yourself, how you treat others.
It's easy to get lost self-pity, I've been there. The only answer is to pick one hard thing where you lack skills, and work on it. Just one. For me, it was being more outgoing. Anyone asks me to do something social, the answer is always "yes". Someone wants to come over for dinner? "Ugh fine", in my head, "Oh that would be amazing!", to the other.
You are where you are potentially because you chose the easy path, instead of doing what is hard.