I'm interested that you don't mention a partner or family here.
In my experience, feelings of low self-esteem/negativity often come from bad relationships with your parents or your partner - nothing like the drip, drip, drip of nasty undermining comments to make you feel really bad about yourself and then feel awkward and self-conscious in social situations (and then attract people who are mean and take advantage of your vulnerability).
I often joke that I had no social skills until I was about 28 - which is sort of a joke - but not entirely.
As a teenager I felt terribly uncomfortable and this definitely continued into my 20s - though I doubt people would have noticed. I was good at projecting false confidence and giving the impression that everything rolled off me etc etc
Obviously b/s.
I had quite a bit of therapy in my late 20s and learnt to accept myself - from your post, you sound incredibly hard on yourself. I think this is key - it may be a cliche, but why should other people like you if you don't like yourself. Also, as another poster rightly said, don't ever feel you have to do anything you don't want to - unless you'll get into serious trouble for not doing it.
As a much older person (41 going on 42) I find friendships much easier and have learnt to be more accepting and forgiving of people. I think I expect less from people - they have their own lives and their own issues - and enjoy the time I have with friends.
Also, avoid facebook until you feel more confident. It has its uses (ie wasting time/chatting to odds and sods when you've nothing else to do) but please do not go there for validation because you won't get it.
Enjoy spending time with your kids, get active in your local community, think about studying, working or finding a different job. Read more books, listen to your favourite music, cook, make stuff, be creative - what ever it is that makes you feel good.
Build your own confidence and then you will find people will be looking to be friends with you.
Sorry if this sounds like a lecture, you just sounded so upset and I really felt for you.