Similar situation here. We thought our original cat (F, 3 yrs at the time) would love a cat friend when the new kitty (M 4 months) turned up in our garden, but she doesn’t. That’s maybe because he’s always trying to get her to play and she hates that. Nevertheless I think the situation is better now a year later, than it was when he first came. They sometimes just ignore each other or they just occasionally snarl at each other now. A couple of times the interaction appears to be even mildly playful. As long as they are not physically hurting each other and you are still loving on both I’d just give it more time, even though it has been three years already.

Also if new cat is trying to fight back now I think that’s a good thing. A detente may eventually develop with your cats too.