Some antidepressants really fuck with your libido. So this indeed could be your problem, although you being able to orgasm by yourself doesn’t match what I’ve heard.
Without trying to sound rude, I’ve been there myself: This could also be caused by excessive porn consumption and having your brain being wired in a way in which masturbation works well and sex being difficult for orgasming, despite feeling good.

I guess there are multiple factors. I think the biggest one is just the personality of the person. But starting taking hormones is basically a second puberty for the body. Most trans folks do not become as irretable as a person in their puberty but if one has a predisposition to it, a transition may trigger that a bit.