There’s a great site called The Friendship Blog that I go to sometimes (when a friend is being maddening). It’s brilliant, and it has forums, too. They have sections on Making Friends, Keeping Friends, etc.
When I’m feeling the way you are I remember that I’m not for everyone! LOL!
Also, consider your age. So as your twenties progress, you could also feel left out because everyone is getting matched up, moving, working. In your thirties you make friends through your kids. Forties, you have a handful of keepers LOL. Then your friends die and you have to make friends of ALL ages.
So cut yourself some slack. Call up your friends and make dates to get together. And keep in mind that some people are just clueless. But remember ~ you picked them!