Sounds like you are done with superficial relationships and probably feel this loneliness, even when in groups, because you are seeking true intimacy with another human being. Therefore being known for your true self, worts and all, and the acceptance and love that comes from living with a loved one and working on a relationship from day to day and year after year.
Sounds like like you are ready to settle down. You sholdn't have a problem finding a partner. Good Luck you'll find what you want eventually.

I have felt like this before. Mostly it happens because I'm in a sour mood (bad day, poor test, etc), not because people are leaving me out.

No Knocker.....it's not just you. I can be really happy on my own for weeks at a time & then it just hits me. Usually on a weekend when my kids are with their dad.
I pull on my boots and head off to the hills with my dog, that normally sorts me out x