Better to be true to yourself, but perhaps seek counseling before you decide either way.

You gotta do what's right for you. You won't be truly happy if you stay closeted. And if she is truly a great girl, she'll still be your friend, and accept your realization. Best of luck!

Sounds like you have some more soul searching to do. If you're not sure, you may want to talk this out with a trusted friend, family member, or maybe even a counselor so you can sort your thoughts and feelings out one way or another before making a decision. Also, if it DOES turn out you're transgender, then staying closeted would only serve to break both of your hearts. You, because you're in a relationship where you can't be yourself, and her because she will eventually discover her relationship is based on an illusion, which she may not have even wanted you to try to keep up (and wouldn't if she's worth a damn as a human). Either way, since you're uncertain, the first step is to get to know yourself better to be sure (knowing yourself is of EXTREME importance regardless) and just be you. If you aren't transgender, then carry on. If you are, and your girlfriend is really as great as you think she is, she'll understand and support you, even if you're not romantically compatible anymore. If she's not supportive, then that sucks, but in time you learn that the only people at all worth having in your life are those that accept you for you and will only support you in striving to be your best self. Either way, I wish you much luck and happiness!

You have to be true to yourself or you'll resent her and end up hurting her later. You'd be surprised how many relationships survive transitions. No, it's not likely. But no matter what, honesty is better than romance with the wrong person.

First step at this point, IMO, is to a gender identity therapist. Reason being is, they specialize in helping people discover and navigate the nuances of the trans spectrum... crossdressing to Tranvestism to transgender to transsexual and all that goes in between and beyond. They, in theory, will help you find your definition... and if you decide to transition at some point, to whatever extent you want to transition, you will need a therapist anyway. One thing that people tend to forget is that being transgender doesn't require the individual to transition, though in most cases, it's the recommended treatment for gender dysphoria. Being honest about the disconnect between your identity and your body can help your partner understand aspects of your relationship and also help them understand what kinds of things you need to help you. Your girlfriend might claim to be exclusively straight, but she might find that in regards to you, she is more demi or sapio sexual, and is in love with your brain and emotional expression far more than the picture you display on the outside. I agree that you need to discuss it, the sooner the better, because you both deserve to be with someone that you can accept and who can accept you. You're not breaking her heart by telling her this information. You might be informing her that the relationship can't continue in the ways she imagined... BUT having the discussion, you might find out that the "dealbreakers" she has are about things like wanting biological children between herself and her partner (which you can still do) and you'll end up having one of the most loving, supportive cheerleaders for the evolution of your life.

Stay in the closet. It’s nice and cozy and you already have a nice girlfriend.

I feel your struggle... I came out as trans to a boyfriend a few years ago and it ruined the relationship... but it also made me realise that the relationship was never right because I had inadvertently built it on a lie. I'm not saying that it will be the same for you, but you owe it to yourself to be honest about these things and hopefully you and your girlfriend will find a way to make peace with the situation and work out a way to continue either as friends or as lovers.

Stay closeted

Damn bro, that's ****** tough. But....I think that if you're unhappy with how things are, then In the long run being depressed about it will also affect her, and it will be something neither of you can help. Rip the bandaid off, you can do it.

I'm no expert in trans issues but I am a 100% straight woman like your GF. I guarantee you that if my late first husband, or my current husband, told me they'd realised they were a woman, I wouldn't go anywhere. Would I date a transwoman I just met? No. But for someone I was already in love with, heck yeah I'd redefine my sexuality to "pan" or whatever. Not saying your GF is guaranteed to stand by you. I'm just saying it's totally possible she will. IMHO if you don't tell her, a secret that big will poison the relationship anyway. I think being open with her gives the best odds at happiness.