Well, I can honestly say I know exactly how you feel. I have never been officially diagnosed with depression, but have always had a feeling. It kind of stems from previous relationships in my case, but also with the family as well.
I haven't always been a very sure person, only recently have I realized that there's no point in trying to hide myself from the rest of the world. If people don't like me or what I have to say, then they don't have to talk or be around me. I used to very shy and put up with a lot while I was in middle school. I'm kind of a bigger guy, so I always tried controlling my temper. One day I didn't and at that point people left me alone (meaning they didn't pick on me anymore). I'm not saying that you need to snap on people, but you don't have to put up with some of BS people might throw at you.
In regards to trusting people I can't give you any advice there, except that you're going to have to break out of your shell and give someone a chane. I have and yes I've been stabbed in the back, but I got over it. You have to find something to do that will take your mind off it for a while and then next thing you know, you've totally forgotten about it. For me, I just listen to some really heavy music. I guess, in both cases you'll just have to take a chance....there are some good people out there and it will take some time to become comfortable with them, but you will. I mean if I can, anybody can.