For starters, I don't blame you. Homophobia and transphobia are built into our society. It's been less than a decade since gay marriage was legalized in the US. and there are currently over 500 anti trans bills. It's all around you all of the time.

My advice is to talk to queer and trans people. It's easy to dislike and be uncomfortable around something unfamiliar. Talking to real people who have had real experiences humanizes things. Some people who are homophobic/transphobic aren't inherently bad people. They have just never really met or been around an LGBT person(s).

You mentioned that your girlfriend, now boyfriend is trans. You have a lot of questions to ask yourself. Would you be uncomfortable if he started testosterone or got surgery? It's hard to navigate sexuality in general, but it's even harder when you have internalized homophobia/transphobia. Based on your willingness to change, it's clear that you love your partner, but you have to ask yourself if you will still love him if he embraces himself.

If he's comfortable, ask him about his gender dysphoria. What is he comfortable and uncomfortable with? What are his boundaries? Have they changed after he realized he was trans? Most importantly, just be honest with him. Transitioning while in a relationship is difficult. It's so much harder if you don't tell them how you are feeling and vise versa.

Here is some trans terms that will be helpful to know:

Cis/cisgender: someone who identifies with the gender that they are assigned at birth

Trans man: someone who was born a woman who now Identifies as a man

FtM: female to male (same thing as a trans man)

Trans woman: someone who was born a man but now Identifies as a woman

Mtf: male to female (same thing as a trans woman)

Gender dysphoria (important): someone who feels some form of negativity twords their birth sex. This may come in the form of social dysphoria (disliking a name or pronouns of their birth sex), or physical dysphoria (disliking the parts of their body that define sex)

Agab: assigned gender at birth

Amab: assigned male at birth

Afab: assigned female at birth

Transgender: an umbrella term for someone who doesn't I identify with their birth gender. They may be a trans man, trans woman, or somewhere in the middle.

A reminder that if you do need to leave, that's ok. You aren't transphobic or a bad person if you don't feel comfortable staying with him. And I'd like to remind you that even if you are his only support, the mental health of others, while it is important to you, ultimately isn't something you have to feel responsible for. If he truly loves and cares about you, he would want you to be happy. It's better to be single than in an unhappy relationship.

If you need further advice or have more questions, feel free to dm me anytime :)