It's hard to be positive when you're surrounded by negativity. The first step to being a happier person is by thinking positively and have a positive surrounding!
I've been talking to some people who message me anonymously on tumblr. I said no to one request they wanted from me and they said, "Appreciate the people in your life, remember what happened in the past?!" I know who this person is and she must be in my life for some reason. I think the reason is to help her and other people move on. Yes, I wasn't the nicest person in my past but everyone has a limit. No one is going to be the person who says "yes" to everything. You can't go through life being reminded of your past. Your past doesn't define you. No one can say they were the exact same person years ago. If they can that means they never changed. In this case no change is a bad thing.
Also I wanted to point out that if someone points out your past and won't let you forget it, they're not your friend. People who care about you will want you to change and be a happier person. The people who drag you down like that needs to work on themselves and you can tell them that. Everyone has their own definition of life, but we can all agree on that growth is part of it.
This quote I thought of, probably read it from somewhere long ago, put me in a more positive place after this experience. "When a flower changes you don't ask why they do. It just blooms." Be the flower, not the weed.