Is it okay that I've been talking to a girl who I used to like but now I'm married to someone else?
I think it's fine as long as you're just trying to be her friend. Why did you stop liking her?
I was no longer interested in her.
If you just want to have a friendship with her, that's fine. If you have other intentions of a physical relationship then that would be wrong because you're married.
Yes, I think I want her back now.
This would really hurt your marriage. You made a commitment to your wife and unless you get divorced, you can pursue another relationship. I think you have a choice to make about your feelings and what will make you truly happy.