This is very true, I've had what I thought were friends but every last one of them was using me for one thing or another...free website & design/business advice, free babysitter/ride/someone to hang out with until I can get the hell away from this town and back to my real friends.
Unfortunately every single person I have ever known has used me and dumped me (apart from my husband) and I think what Pella said is very very true for me now, the slightest bit of attention and I obsess over it in a "Ooooh maybe they want to be friends" etc. I get too attached too fast, desperately hoping that I might make a friend so when yet again they stop speaking to me it's another harsh blow and I get ever so slightly more skeptical about people. Right now if someone started being friendly towards me I'd still have that initial rush of "yay they want to be friends" but then that little voice would soon turn to "no they don't they are an asshole like every other person you've ever thought that about and as soon as they have whatever it is they want from you they will stop speaking to you"
The only person who has ever proven that little voice wrong is my husband, even after we were engaged I wondered if it was some elaborate joke, that he'd dump me and laugh at me for believing all the things he said. Even after I moved in with him I had this little voice warning me that at any moment he was going to use/abuse/leave me like everyone else, it took a long time for me to get past that and realise he was genuine and wasn't like everyone else.