I've worked hard to overcome negative thinking. I have been working at it diligently for about 2-3 years and it still takes work I'm definitely much better than I used to be though.
I spent years of my life being depressed and way too much time thinking about how people had done me wrong.
I am amazed that I almost never find myself thinking about negative stuff anymore.
I even try to spend some time thinking about what I am grateful for.
It takes some time but you get better and better at it until the pendulum swings in the other direction where positive thinking starts to become the norm
A few of the things that have helped me.
youtube meditation and hypnosis videos. There's also tons of stuff on the web that isn't on youtube.
Motivational books, from my library, both books and cd's. Many libraries even let you download books and audio books right from their website these days.
Motivational books on youtube, I can't believe how many books and cd's I find on youtube. Some are copies of the cd some people will actually sit down and read a book and post it on youtube.
Search for "full book" and then start to narrow down your search when you find stuff you are interested in.
Youtube will start to help you out by finding related stuff.
"The secret" was very motivational. Even if it isn't true it helps me to think more positive and I have become more positive.