If you have a partner it's the less likely outcome that they'll reject your new gender identity. Gender is a spectrum and we're all a lot more interesting than just what our gender is anyways. Someone who was attracted to you was attracted to a lot more than just your gender expression so, on the whole, rather little about you is changing when you come out as trans.
I am not trans so I can't talk to that realization but I am non-conforming and coming into that identity was mostly a relief. I had to cloak a lot and found myself pretty distinct from a lot of the stereotypes about men - I was married at the time and my partner didn't bat an eye.