Still Cheat

So I still cheat and I never set out to intentionally cheat. My husband doesn't know.  I have been seeing the same guy for a year and almost 8 months. Sex is beyond incredible.  He recently had his gf whom they've only been together for 14 months move in.  He still sees me obviously  it's been harder to do so  as the gf is super needy and clingy. He's a bit if a sociopath and narcissistic. The gf knows he was sleeping with me and another woman but has chosen to remain with him and moved in at the end of June. I am his side dish which is fine but he says at some point all side dishes will have to come to an end because he needs to change his behavior things are going good for him.  I'd  like to ad the gf is so ugly she could make a freight train take a dirt road. So I wrong to believe that a leopard never changes it's spots? He's  54 I am early 40's same as his gf. He has been cheating since he was in his teens and was cheating on his wife before they married. So it's been 37+ years he's been cheating. Any suggestions on how to get him to keep seeing me and keep the relationship going as friends we have many things in common but have little time these days to chat when meeting up. I believe he may ask her to marry him come September on her birthday so he will have someone there always that will put up with the cheating and never leave.