You're doing pretty much all you can do. Funny how a mother can essentially abandon her child for some guy, although guys do the same. I'm pleased that you have your child so much. It's great. Sets the precendence that you are the custodial parent. Has she been paying you child support?
I think you should just go with the flow. You can't do anything about her bad decisions so just be the best father and person you can be. It's her loss if she doesn't choose to try again. It may be your loss if she decided to try again with you...who knows.
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer