I definitely went through those emotions, especially when I was 19. But then I thought it was a "phase" and I'd grow out of it, too. The thing is, there are a TON of emotions that come with being gender fluid or trans. I am now 11 yrs post-op and to this day still feel kind of like both genders. And I think that may be a key thing here - that not knowing what gender we are bit - because perhaps in reality gender is only a mental/social construct and perhaps we humans are much more fluid in gender, sexual orientation, and all of that than we realize. It could explain a lot.
One of the other things that is a challenge is this: just the very notion of being able to transition can be a factor. That we can think, "I CAN be a woman, if I want to." is exciting. The possibility is there, but does that mean you should go for it? Who knows. That choice always comes down to you.
Perhaps spending as much time as you can in girl mode will help you figure things out. You should definitely join a real world support group. Nothing else even comes close to finding out where you fall on the gender spectrum than hanging out with a bunch of similar folks, especially when they're all over the spectrum.
Lastly, are you seeing a counselor or psychologist about it? If not, find one who is experienced in transgender issues. It is usually pretty easy to find one via a support group, or vice versa. Look into both.
Ultimately, the decision to transition fully comes down to what is right for you. Only you can make the choice, but make sure you make the choice based on a solid decision and not a spur-of-the-moment one.
Best of luck to you!