I'm going to let you in on some little-known secrets. Research has shown that upwards of 70% of non-straight ("queer", as I refer to them) trans women readily admit to experiencing sexual arousal (in the form of penile erection) in response to crossdressing, forced feminization narratives, and other simulations of being feminine/female. Sexual arousal in response to crossdressing is, of course, defined as a paraphilia (meaning, simply, an "atypical sexual propensity"), and is known as transvestism. In addition, about one-third of queer trans women display at least one additional paraphilia, most commonly masochism or sadism, but also specific object fetishes, diaper fetishism/sexual infantilism (I'm aware of two cases of this, one from here, and the other one of whom I know personally), and many others. It is important to note that paraphilias tend to cluster/co-occur. Lastly, queer trans women generally report relatively minimal gender-atypicality in childhood before puberty, and recent brain imaging studies, in accordance with this, have found that pre-HRT queer trans women have male-typical brains (whereas their straight counterparts do not).

All of the above is hard, raw, quality scientific data. In addition, I want you to know that your inquiry is one that is seen on transgender discussion boards and such all the time -- it's very prevalent. There is a natural and fairly obvious, but extremely controversial conclusion that can be drawn from all of this. This is essentially that queer trans women are not experiencing neurological intersexuality (i.e., "female brain") as the basis of their transgender feelings, but rather a powerful paraphilia, which has been described scientifically as "autogynephilia" (and transvestism technically being a subtype of it, with other exemplary subtypes being body, behavior, and self-concept as opposed to clothing), that produces their gender atypicality and dysphoria makes them wish to change their sex. Whether you choose to go with this conclusion, as I personally do (after years of research and qualitative exposure to countless trans women), is completely up to you. I simply want you to be aware that it exists, as I feel it may answer some of your questions.

In regards to feeling like a girl internally, be careful to observe the difference between truly knowing what it is to feel like something (i.e., "like a woman") and making the possible mistake of thinking that you feel like a certain something because it engenders good feelings which in turn feel "right" and "natural" to you. Few queer trans women are capable of properly making this distinction. Just to heighten your awareness a little bit here.

Now then, with all of that out of the way... if it is indeed a "fetish" (technically incorrect -- "paraphilia" is the right term), this does NOT mean that you aren't "trans" or aren't "a girl". There are countless individuals (about 65-70% of all trans women, to be exact, with the remainder being straight) who show this profile of cross-gender desire and who transition and do just fine and are very happy in their new roles and lives, including myself. And as for being a "girl", that is a socially-constructed term. Cis lesbians show behavior and brain morphology that is masculinized and is arguably overall more like boys than that of girls, yet we nonetheless apply the label of "girl" to lesbians (except for the fraction of them who reject the label of "girl" and decide to call themselves "boys" instead -- i.e., trans men -- of course). Whether you are a girl or not is based simply on what you decide for yourself to identify and conceptualize yourself as, and of course to live as. Having a female-typical neurology is not required to be a girl either definitionally or in terms of success and happiness in one's life and social gender role, as lesbians show us.

As such, what this should come down to, and whether you should transition or not, is based on what you feel would be best for yourself. Do you feel like you would be happier and more comfortable being a girl? Do you feel like would be less dysphoric no longer being a male? Do you think that the net improvement in your well-being that this change would result in would outweigh the immense challenges and possible negative outcomes of transitioning? For instance, issues with family/friends, the costs of and stress related to transitioning, perhaps being less attractive/not passable and having a more difficult time socially and/or sexuoromantically because of it (a sad but true possibility to consider), and all the rest? These are not to be taken lightly... there are people who have killed themselves because transition did not go as well as they hoped (Google "hopefulkylee" for a really sad, tragic example of this -- she was so happy and excited to start transitioning, but then she lost her girlfriend and family, was struck by the reality that she would never be very attractive or passable as a woman, and ultimately committed suicide two years later). But for most, it seems apparent, they are happier transitioned, even with the often many drawbacks. It just comes down to you and your personal circumstances.

Hope that was helpful to you. Good luck! ^^