I met someone prior to dating my girlfriend and can't seem to stop thinking about her. I honestly do not want to but I just can't stop it. What can I do?

Aha, the truth is out about (some of the reason) why you are so unfeeling toward your girlfriend.
What you should do is tell your girlfriend the TRUTH and break up with her. You need to untangle your mind for a while. And put the other woman out of the picture until all this is DONE.
Your poor girlfriend! You are too much.

I really don't think it's all that unusual for a young guy such as yourself (Females too, for that matter) to be both shallow, and so unfocused in thoughts, and actions, and easily distracted. It's mostly inexperience in dealing with your own feelings.

Always stay within the boundaries of good behavior.

Think before you act or speak.

When your thoughts wander, physically get up, and do something productive or useful.

Practice these thing in your daily life and you will get good at being cool, calm, and in control of yourself, focused on things you need to, and not on things you don't.

You are in the middle of growing pains, and it may last a while.

You need to be honest with your girl friend and stop blaming her for your straying thoughts. Leave the current girl friend along and get your head straight
https://www.askmehelpdesk.com/relationships/am-too-stupid-808760.html

I think its pretty apparent when taken in the context of both questions you are LEARNING things about yourself that you wish to change.

Take your time. First decide what do you want. Then do it and also stick with that decision.