ya dont worry too much about depression meds and anxiety meds stopping HRT, it may cause them to adjust it but lots of trans people are on similar, I had concerns with my bloodpressure but right after it started coming down my doctor was ok with starting hormones, and it wasen't a bit high it was near heart attack at all times high.
As for sexuality I find it easiest, especially when in the middle of transision and really in life in general, not to worry too much about labels. I am a woman, I am almost exclusivly attracted to women I don't think its worth digging harder into that sexuality is not as set in stone as many people think its best to just let it happen.
As for your significant other, it may be hard now and will certainly be hard later but don't count out any possiblites, many people make definitive declarations when they are confused, lost, sad or just angry, and in the case of a romantic relationship and a gender change it can be even more severe as it can easily cause them to question not just the relationship but there own sexuality and that can be terrifying so give them time and space, try not to force anything and if something happens talk to someone else, even if its just a post here, a outside perspecitive may help reveal why and how someone feels and makes dealing with it easier. But no promises this could end your current relationship but if your careful and let them take the time they need to proccess this there is a very good chance you can at the very least remain friends.
Serena