I have been knowing this girl for almost 10-15 years. We have dated off and on for three times. This time we broke up because I was not as much available to her. It is not that we had a lot fight or anything like that. Now she stated she found someone and that she is very happy. What do I need to do to get her back or what are my chances? As of right now she does not even want to communicate with me anymore?

Think about it, even if you get her back, how do you know she won't break up with you again and find someone else?

I'm sorry to say, but I don't think you'll be able to get her back. She found someone else, so obviously she's moved on with her life.

No more contacting her. It's time for you to move on with your life too.

If she is very happy and she does not to be with you I suggest you respect her wishes and leave her alone.
You can not force someone to love you or want to be with you.
Apparently ,you had a chance and it didn't work out.
Allow her happiness and you find happiness with another.

It didn't work before, and chances are it won't work again, so leave her alone, and get your own life together, and make yourself happy, without her.