How can I forget the mother of my child?

Not really realistic as you will still have some sort of contact with them due to your child.

You can't... you have a child between you.

You never will, so learn how best to deal with her. Easier said than done sometimes, I know.

Only talk to her if it is about your child that's the only thing you can do.

You pay her child support, don't visit with her, just go pick child up for your visits