Practice thinking positive thoughts....seriously. They don't just come by themselves, you sort of have to kick the negative thoughts out of the way and let the positive ones rein supreme. But...that's easier said than done
Maybe look into some therapy to help with the thoughts. It's tough when you are used to thinking negatively. Sometimes it feels like it's all we know how to do...and maybe it is.
What I do...is I journal the times when the neg thoughts take over...I write it all down. Then when it turns out that I was TOTALLY wrong about the situation (which is usually the case), I write that down as well. I make myself look at those journal entries a few times a month. Just to remind myself how wrong I was before.
That's something that's helped me gain more pos thoughts than negative. I dont' have pos thoughts all the time yet, but I've gone from probably 90% first thought neg to maybe 30% first thought neg. It will take some effort, but I think with therapy and some practice (it REALLY is very important to practice every day in the beginning) you can make a difference. Good luck