You know when the same thing keeps happening and you have to accept it's you, not them? I've always known that really, going back to school I found friendships difficult, and always felt on the fringes of things. My parents are very much a couple who do everything together, and whilst they know people and are polite and friendly, they never had good friends. I've never met any of their friends in 50+ years, that part of life was never modelled or considered important.
After I married, quite young, I was happy to concentrate on career and home with DH. It was a relief not to "need" friends. He didn't have a lot of friends either. I had work friends, but nothing ever translated to outside work.
When I had DC, I "made" myself socialise for their sake, and had 2 nice groups I did play dates and the occasional mums' night out with. They were a rock at the time, but fizzled out once DC started school, although I believe some of the others are still in touch.
The primary school years were awful for me. I really struggled with the school gate. It was like being back at school, although I was happy with DH and family, generally, I felt really out of place every time I made any attempt to chat with other school parents.
I joined a hobby group and finally found what I thought was my "tribe". I spent a lot of time with them on the hobby, but also days and evenings out with those people doing other things, and as couples with all our spouses. They became a huge part of my life.
Then DH died, and they all disappeared overnight. Even while I was planning the funeral, all contact dried up. They did come to the funeral, but I've barely seen them since. Initially I did make an effort to contact them, but responses were so luke warm I gave up. I still have no idea what happened there.
Afterwards, I found another group, mostly single people, who became a family. We relied on each other for all the practical and emotional support you might otherwise get from a partner. I.e. the group fulfilled all those roles in one way or another, rather than having one person. I'm still very much involved in that, but now have a new partner and I can feel the dynamic changing and things drifting off...