From what you've described so far, forgive me if I'm wrong it's hard to determine that what she is doing is causing much problems at all first. If you are leaving out specifics other than the comment that "because he has a gf now", it would help to know exactly what she's said or done to really make you out to be a problem.

As for the time the dad spends with the daughter, first question comes to mind: you said "separated" for 6 years. Are they divorced? Is he still married to her? All sorts of problems come to my mind in thinking if he's still married to her. If so I'd wonder why they haven't divorced and you've got potentially bigger problems on your hands with him. Also, if they are divorced there must be a custody/visitation order, and if there is, he can get help to enforce this so that he gets the time he has a right to with his daughter. If they are only separated, that could get complicated because it's not always required. With regards to his "not forcing her" to be with him, I agree with the poster that this should NEVER be left up to the child. It places undue pressure on the child in that every weekend she/he has to choose between the parents and this is never a good thing to do. He needs to step up tbh with the daughter but that being said, you can't do anything about it being the gf. Heck even if he married you it would be pretty much out of your hands on how he handles it.

She could be making things difficult for him, I can see that but unfortunately you're hands are tied in regards to this and your only option really is to handle yourself as best as possible in view of others and the ex. You can only control your own actions and behaviors, and if you're being good to the daughter, you're doing all you can.

Sadly you have no real say in the matter, you can only give advice to him and talk to him about it. I would recommend not even being there when the ex is. JUST NOT A GOOD SITUATION at all. She's not going to ever accept or like you. Why should she? you're a replacement, no matter how bad she was to him or how badly the marriage went, you'll always seem like that. She'll probably never have anything great to say about you. Best case: she can learn to respect you but even that just comes down to you not confronting her on anything to do with the child.

you're in a tough situation, trust me, I understand.

Hope it gets better *hugs*