I don’t know if this is the place to go, but I can’t get help

I need help. I have an amazing gf and she is perfect and I can’t stop talking to other girls online and watching stuff online. We are getting older and I want to give her children. But I can’t stop and because of that I can’t perform with her. I hate myself and I am so weak and she deserves so much better. But I’m selfish and can’t let her go. I really don’t know what to do.