Hi, I know exactly how you are feeling, I was 18-19 when I started transition also, and it was very confusing. I had been reading about transsexualism since I was like 16, and thought I knew for sure thats what I was, and I was for sure going to transition, which I did. But at first, I was afraid that maybe I was'nt really a transsexual, that I just fancied the idea of being a girl, just based on the fact, of being around my sisters, and other girls, and the general image of girls, and the whole objectification thing, and there were times that I was afraid that this was really a sexual thing, and that basicly all i did was forced my mind to fit the profile of TS'ism that I had been reading about.
But after a left home, and transtion was rough, I got into theraphy and started ironing out all of these doubts and I eventually knew for sure that I was doing the right thing and that I was 100% sure. But you are right to question yourself, and its best to transtion and start making physical changes on a 100% certanity instaead of a 95% certanity. Thats why the theraphy is good.
I was the same way, at first it took me the longest time to refer to myself as a transsexual, and it took even longer for me to feel comfortable refering to myself as a girl. I thought I would never pass, and thought that it would be obvious that I was just trying to "fool" everyone, and during the early stages when everything was still sort of confusing, I was always afraid that maybe I was'nt doing the right thing. But theraphy really helps with all this.
As you actually begin to transition, and and your actual true self starts to emerge, a lot of these doubts will start to disappear and you will start to feel comfortable with yourself and what you are doing. Eventually you wont even need the labels anymore, you will just be you. I'm 27 now, and have been transitioned for quite sometime now, so the feelings you are having are complelty normal, and there nothing wrong with questiong your own motives and motivations.
-pass-