This is really rough. All I can give are my own answers.
Aside from the ethics, there *is* no way to change the mind to match the body-- reparative therapy doesn't work for trans folks anymore than for gay folks. Changing the body is the most effective treatment available to releive dysphoria. You might also try asking her which she would consider to be more undermining her status as a person, mastectomy or electroconvulsive therapy. No one should be under any obligation to "change their brain."
The way I explained the desire to physically transition to my partner was to say that, if the world would accept me as a man and treat me as a man without surgery or hormones, I don't think I'd need them. Now, that's just me. Everyone has things about their body they don't like, and I think I could cope fine with being a short man with breasts and no body hair, if I were just a *man*. But gender is a social concept, which means that, in order for gender to "work" (not to be valid, to be functional), it must on some level be recognized by society. Every dau as a woman feels like one great big lie that no one will let me stop telling. It sucks.
I wish you luck with your partner. My wife, also lesbian, has had some of the same issues, bu we're doing OK at the moment because she's willing to wait and see where this goes. But if your partner doesn't want to try, would really rather loose you than consider the possibility she might be happy in a relationship with a man, there's no help for it. But that is not your fault. Unfortunately, the people around us are often hurt by our coming out and our transition. But we don't do it to hurt them, and we're not responsible for inflicting their pain. Doing yourself irrevocable harm to avoid causing pain to others isn't a viable answer. Society and the stupid gender system is responsible for their pain, with a large dash of chance thrown in for good measure.
This guilt isn't yours, don't carry it.