The whole 'am I transgender or I this just fetishising' question is something people struggle with a lot.
The thing is that going through a male puberty is a sex drive overload, that's just nature for you. It would be unusual if you didn't confuse sexuality and gender together, especially growing up.
Personally I don't think that if someone's 'cross dressing' only exists during sex then that automatically means they aren't trans. If during sexual arousal you exhibit more classical behaviours of a female then something is causing that. I don't mean being submissive, or which partner you choose. It's difficult to explain what I mean but sex is more than those two things, there is a level of communication going on between yourself and your own body which is usually very honest. The rest of our daily lives we hide that for social reasons, sex on the other hand is usually done in private. So someone who only displays this behaviour during that time may have a whole number of reasons for it being that way. Yes it could be fetishistic, but not necessarily so.
That's just a general thought, not saying that yours only displays during sexual instances. Just saying that if it did, that doesn't automatically mean it's fetishistic, and that there is not a deeper identity thing going on.
In my opinion, you typing your post, in the manner you are speaking, just sounds like a typical trans person to me. You are clearly in normal (non sexual) mode, typing about your current situation, your past, your grandma. You don't in anyway sound like you are conflating this with sexuality issues.
Figuring out if you are trans is like algebra, you rule other things out, and it's what you are left with. It might be that you meet someone you are attracted to and love, that's all great but something's just not right. You have sexual moments like most people, again something is a miss. You might crossdress, something's still missing. You do your usual life, go to work, school, socialise, people treat you in a certain way like a man or a woman, but it just doesn't seem right.
Being trans is about engaging with the world and yourself in an honest way. Sexually, socially, appearance-wise. For someone for which this is a purely fetishistic thing, there isn't something missing. They're not going to feel that twinge of disappointment if they have to get 'uncrossdressed', or if they get 'crossdressed' but have to pad out their bra. Or that they 'have' to hide their breasts for FTM. It's not the act of doing it, it's the fact that you 'have' to hide things or fill things out which is the problem. Every has different symptoms, they might not cross dress at all.
For trans people these little missing elements are dotted throughout different aspects of their life, it might be glaringly obvious, it might be subtle. Talking about it openly with some sort of therapist or friends and family usually helps get to the bottom of it. For the purposes of finding an answer, or even a vague one, concerns with passibility are irrelevant, as are worries about what this or that person might say, what it might mean relationship/employment/family/friend-wise. You can deal with that stuff later. Adding that into the mix just sends you round in circles.