Ended long term relationship, who gets the cat?

I ended a 13 year long relationship early this year and am finally moving out. Things had been toxic and not healthy. I wanted to end on good terms but unfortunately that is not how things are playing out.

While we were together, I found a cat outside our house. My partner helped me catch her and then I took her to all her vet appointments, did all her medication, cleaned up after her and made sure she had food and water for years. My partner paid for veterinary care and occasionally helped out with her. I see her as my cat though and we’ve always sort of said she was because my partner already has 2 cats and we each already had our own dogs. My question is should keep the cat? I love that cat so much and thought she would be going with me but my ex is certain she has to stay in the house with her. She says she’s bonded to the other two cats and taking her would be cruel. The cats tolerate her and don’t fight with her or anything but I wouldn’t say they are bonded, they don’t groom each other or anything like that.

I believe she would be ok going with me but am I being cruel wanting to take the cat with me and my dog? Just looking for some advice to try to come to terms with the right thing to do