The Past Will Always Define The Future

I'm beginning to not believe in friendships or relationships of any sort - at least not with reference to myself. This is because things keep happening that seem to show that people I thought were my friends just aren't.
Imagine if you will, a girl who has had 3 long relationships, all of which have ended with her guy cheating on her. What hope does she have for a relationship now? All of her experience points to the fact that guys always cheat, so she's gonna have trust issues in any relationship in the future. Alternatively, maybe she will think that there is something wrong with her and that all the cheating is her fault - that she somehow brought it on herself because she isn't worthy. Either way, her past experiences are gonna make her future more difficult.
And so it is I feel with my current situation. Since leaving the US and returning to the UK I have discovered that many people I thought were my friends are in fact not. People are just dropping me like flies. My facebook friend count drops as people de-friend me. They just stop talking to me. One moment e-mails are going back and forth, then nothing. Several people all have a number on which they can call me free, but do you think they ever do? If I hadn't called several people myself, I doubt I would have ever heard from them again. You know someone for 6 years, go to their family's funerals and things and then you leave and that's it - never spoken to again. And it's not just in leaving. I've had roommates steal hundreds of dollars from me and yet somehow expect me to still be their friend. And like I idiot I was! And I've given rides to someone for years all over the place without ever asking for a cent, and then when I needed a ride in turn I had to pay $40 for it. Are these people really friends? People who run from you like the plague as soon as you can't dig them out of the situations they have themselves in any more because you are now in another country. People who will happily have you mail them things from England and yet can't even send you a postcard in the mail themselves. Since being back I've sent many packages Stateside, yet I've had no mail at all myself. It seems quite clear I'm not missed.
And so these things that have happened - these "so called friends" using me or forsaking me over the years cannot help but shape how I feel about things now. And this is not good. Why? Because I do have a few friends out there - real, true, solid friends who wouldn't forsake me and will always be my friends. But all those other people have started to destroy my faith in friendship and make me wary of the friendships I still have. Now I find myself fearing abandonment and lonliness above all else. And naturally this makes me nervous and I misinterpret what people mean, always fearing the worst. I let the attributes of people who have hurt me become associated with people to whom they don't belong. And thus run the risk of a self fulfilling prophecy - making my true friends abandon me because I "punish" them for the "crimes" of others. Because seriously, who wants to keep being friends with a crazy person, who no longer seems to have faith in you or in your friendship?
So this is my dilemma. Friendships - or non-friendships as they have turned out to be - keep defining how I act with my friends of today, and if I keep it up I won't have a friend left!!