Hi @Whaledone1234, welcome to the forum.
I felt like I was in a similar position to what you've described for most of my life. After a serious health diagnosis three years ago I realized that all those people I had offered support to but never got any in return... were nowhere to be seen when I needed them most. I realised that all those years of being there for other people, while it always felt like the right thing to do, was ultimately just being unfair to myself. We all deserve to have relationships with people where there is a level of mutuality. Realising this I did see myself differently, more positively, but I also started expecting some level of connection back from people around me. I wasn't going to accept those one way relationships any more. Yes, it meant I've lost contact with some people, but they were people who were never going to be there for my any way.
In seeing myself of being worth more that I had in the past, it helped me to be more open with people and I have made some very strong connections with people. I now how some people around me that I know I can count on when I need it, and they know they can count on me when they need it. After 53 years of thinking I was the problem of never being able to connect with anyone, I saw that the issue was never my ability to connect, it was my self-confidence that held me back. I'm not saying i'm now this massively confident and extroverted person... so far from it, but I do feel like I have enough of a sense of self-worth to better connect with people and form stronger connections.
After long enough, we just come to accept things the way they are, because they've always been that way. But even a little bit of a change can get us to step slightly out of our comfort zones and stop hiding our real selves from everyone. I still struggle with it, but I feel like i'm in a much better place and able to interact with people on a much more equal footing. So my advice, for what it's worth, is to just be yourself and let people see it. People will like you... just trust that and the people who don't... well they're not the sort of person you want around you anyway. I hope this helps a little bit.
As for things to do when feeling isolated. It's hard. But I always try to do things that keep my mind occupied. Even simple things like watch TV, reading, puzzles etc. It doesn't matter. As long as it's something I enjoy and takes my mind somewhere better. I do try when I can though to do things that involve interacting with other people when I can. Even things like reaching out to others online like you have done here helps a lot. Years ago I used to do a bit of volunteering which was very helpful. I won't lie though, this time of year is always a bit tough to get through, so try not to be too hard on yourself for feeling lonely and isolated. Ironically, it's one thing you are very much not alone in.