Hi Sophie,
Welcome. The "am I trans?" question can be difficult to answer for a lot of people, even if the answer eventually is a resounding yes. You're still very young so, no matter what you ultimately decide, I seriously doubt that it would ruin your life. Regarding your girlfriend, that's tough but actually a lot of married, "straight" women will make exceptions when their SOs come out to them so, all hope may not be lost! That said, whether with this girl or one subsequent, make sure that you put your efforts into a relationship in which you and your girlfriend are mutually supportive of one another.
I certainly can't tell you if you're transgender, no one can, but the fact that you're questioning it suggests that it is the most likely outcome. Cisgender people don't really question their gender...
Anyway, I look forward to your posts. You should definitely bug peope on here about their experiences and whatnot. It might also help to make a list of "reasons I think I might be trans" and "reasons I think I might NOT be trans." Comparing the two could provide insight.
Cheers!
