I, too, have not been involved with a transgender person. However, I have been doing a lot of reading on related topics to my own situation so have some things you may wish to consider.
Probably chief among them is that it is biologically impossible for someone to change sex. Have you considered how this process will affect the health and longevity of your partner and how that will affect you? Your finances? Any caregiving on your part? Surgeries are expensive. Who will pay for them? What about therapies needed after? Will they be able to work and contribute to your household and the associated expenses? Or will their pay be reserved for their medical treatments? What about yours?
Also, many transmen are actually detransitioning. Have you considered whether you could accept the person you are dating if they do so? You indicated you were straight, so what happens if they decide being trans isn't what they actually want? Would you be fine with a relationship with a woman, albeit likely a GNC one? Kids? Not an issue? Adoption or donor okay with you if it is an issue? Could you afford either?
If being self-focused is a problem for you now, have you considered how difficult the relationship will be when you start adding in any of the rest? It seems cold I'm sure to weigh this stuff out, but it's hugely important that you do so when it comes to your future. Give it some thought. Do some research. Maybe try and talk with people who have been in your situation, both good and bad outcomes, to give you some perspective. Best of luck.