Ways to be alone?

I'm writing this post, half as a way of seeking advice, and half as a way to just get things out of my head.

I've been dealing with some pretty bad lonliness recently. I've been dealing with it for years, but recently it's really gotten a lot stronger. I've had plenty of relationships, some serious, others not so much. And i always felt like being in a relationship is something which makes me happy. I relish that feeling of closeness that you get when you're with another person, the feeling of needing and being needed, the security and vulnerability. But for one reason or another (sometimes becuase of me, sometimes because of them, sometimes a bit of both) these relationships never last.
My problem at the moment is dealing with the fear of never having a life partner, of never finding that special someone. I'm 30, i feel like my main 'meeting new people' years are behind me. It's harder and harder to find people, social circles close down, lives get in the way, my 'game' is getting harder to find.
At the same time everyone i know seems to have someone, they're moving in together, getting engaged, getting married, having children, and i don't have any of that. It makes me feel sad, and worthless and depressed, like i've been denied something special. And i know that i am probabaly at least partially to blame for my current situation, for one reason or another (fear and anxiety, throw in a few unrealistic expectations and introverteness too).
So i find it very hard to take pleasure in other people's happiness. I see other people get engaged or other big life milestones and outwardly i am happy and supportive, but inside i am a seething cauldron of jelousy and resentment.
How do i deal with that?
How do i become happy in myself, when im by myself?
How do i reconcile my need for closeness with my fear of rejection?
Just what the fuck do i do now?