I can't stop feeling for this guy I was dating before my current 6 month relationship.
Are you serious? Do you ever want to stop?
Yes I want to stop thinking about him.
I understand how you're feeling, but it's normal to think about what could have happened with previous partners sometimes. As long as you are faithful to your current partner then you shouldn't let it bother you too much.
How do I forget about him permanently?
If you are struggling with intrusive thoughts it might be good to talk with a therapist. It might just take time to get over and nobody know how long that might take.