What should I do. I have a struggle with being faithful.
Is there a particular reason why you struggle with being faithful?
I'm kind of a person who can't forget about his ex-girlfriends
It's not uncommon for people to struggle with getting over their ex-partners, however, it's important to be respectful towards your current girlfriend. If you feel you can't be faithful then it is probably best you end the relationship and take some time to work on yourself. It's far more kind than cheating on someone, which is cruel.
Don't you think I'm doing just fine with that?
Unfortunately, you said you have problems with being faithful so I think there are some issues that you need to address before you can move forward with a new and healthy relationship. It's okay to be single for awhile.