My ex girlfiend dumped me 3 months ago although I am with someone else now I keep thinking about my ex everyday , I don't contact her or have any contact with her but she just dominates my thoughts, what can I do?

Feelings don't just go away. Three months really is not that long. You didn't give yourself time to get over that relationship so you'd be ready to move on.

You can either breakup with your new girl, or stay and hope the feelings for your ex fade. The first option will probably hurt her, but so will the second option. It all really comes down to which you think is more fair to her.