I am not positive of the meaning but it isn't nice. I'm assuming you know what "fkk it" means. "Lucky I don't make you mine" could mean 1 of 2 things: 1) "consider yourself fortunate that I don't make you into my definition of a girlfriend - someone I own." Or 2) "consider yourself fortunate that I am not owning you by dominating in a physical fight"
This can often just be a way of saying one is upset without being serious but you have to know that person to be sure it isn't a real warning.