How do I deal with this trait of mine?

I trust people way too easily, even when I don’t want to. I get friendly with them, they get friendly with me. This friendly behaviour continues for a while, then they suddenly stop talking to me. I want to and try to keep in touch, but it seems like they’re avoiding me. They just don’t take any notice at all.
This has happened to me too many times and I want to be able to deal with it without getting hurt, as I do. Any suggestions?