I actually know exactly this feeling. I don't really have friends, I have aquaintences, I have co-workers, but nobody, other than my partner, that is really a friend.
Once I get home from work, I bury myself in my room, and I don't want to talk to anyone or be with anyone, I just want to be left alone. I, though, should probably say that being in crowds makes me panic. I don't know why. i guess it's just another issue of mine. Funny, that I can go to Disney and be ok for the most part. But even then sometimes while waiting in line, if there are alot of people close around me, I start to get shaky.
I wish I could give you some words of wisdom on how to overcome, but I haven't learned how yet myself. But just know that you are not alone in your loneliness. Even with kids and a partner, I'm still completely lonely.