So turns out I'm a walking cliche. When I first realised I was trans I strongly identified as a gay guy but when it became apparent gay guys had a very bad attitude towards transguy when it came to relationships I considered if I might be bi/pan and decided that yes I was. I actually got a relationship with a bi guy but then she came out as a transgirl. I decided not to let it matter and stayed with her. Only for months now I've felt little to no sexual attraction to her and this has been a steady issue as her outward appearance has become more feminine. I convinced myself it was depression or that maybe I was on the ace spectrum but while away at a conference I realised I felt very sexually attracted to a few guys I saw and when travelling back I realised there was no point trying to lie to myself; I'm gay. I do love/care for her a fuck ton but I realise it's the way I would a best friend. I don't want to hurt her but know in the long-run it'll hurt her more if I don't tell her the truth and break it off. Luckily months ago we opened up the relationship and I know she's been dating a few others casually so I don't think she'll have too much trouble moving on - honestly I think part of me hoped she would just meet someone better and break it off herself. I just wish I hadn't wasted two years of both our lives... I just needed to rant about this somewhere I guess, thanks for listening.
Sounds like you ha s thought this through and know what you need to do. However, I would not beat yourself up over it. Your going through a lot of personal development and figuring out whom you are. This takes time and will be filled with trial and error. You current partner is also doing the same thing. Both of you have your eyes wide open and all the cards are already on the table. Finally, I would suspect based on the relationship dynamics she might already be thinking the same exact thing.
You loved her, cared about her, and learned and grew as a person yourself. That is not a waste of time. Continuing further now that you're in a place where you know it doesn't work for you wouldn't be fair, but you seem fully aware of that. Don't worry about the past. It happened and it made sense at the time.