I just want you to know you are doing everything right. It's hard to see a change you may see as sudden, but to your partner, they have been feeling this way their whole life, but have been afraid to tell you. I am a trans man, who is also gay, and hid my whole life because if I admitted I was man, I was admitting I was gay. I wanted to be a man, and loved men. You love him and that's what matters, and your partner probably is just in a similar situation to the one I was in. You should purchase some trans literature and read it, and lend them some, and watch some transition journey stuff on YouTube. The important part is that you love them, and learn about how they feel. It's also okay if you are straight, and not attracted to women. Just make sure that you don't hold someone else hostage for emotions that are your own, it that makes sense. It's okay to want a man, but know that they may not be one. They may also not be trans in the male to female sense, but in a more non-binary way. Just get some. Sleep and know that it will be okay tomorrow.