I just wanted to add my 2 cents.
I chose to leave my ex H, it's been two years now. We got a dog together just after I said I wanted a divorce but gave him two months with counseling to see if things would work out. They did not. So, we separated. I did not initially take the dog but when he worked out of town I would keep her for a few days. However, I did not do a 'visitation' with her as if she were a child. about 7 months later he finally admitted he couldn't keep the dog so I took her. on occasion he would keep her for me to give me a break or I needed to go somewhere. In no way was I ever inclined by seeing my ex to want to go back to him. I'd been through too much and was/am very happy to finally be on my own. On the other side of that, we do have grown kids and a grandchild so there are always going to be times that we see one another and we are ok with it. We are not those people who can't be in the same room together without giving the evil eye or whatever. or worse. All the same. I have moved on, he has moved on. Neither the dog or the kids or the grandchild would be any motive or incentive to go back to that marriage.
Give your gf the benefit of the doubt. There's a reason she is with you and not the ex.