Don’t worry about still having feelings for your ex. That’s normal. You used to like him after all. And even if your relationship did not work out he is still mostly the same person. But what you miss are his good parts and the good times you had with him.
This is normal and nothing to worry about. Accept these feelings for what they are. Nothing is wrong with having them. It doesn’t mean that you like your girlfriend any less.
Everything I said still applies. Feelings are feelings. You cannot choose how you feel. What matters is if you act on them.