Am i transgender?

So I'm a 20 year old guy, throughout my entire life I thought I was straight, despite the fact that I love crossdressing, but now I'm probably more uncertain than I've ever been.

I grew up in a family of four (my parents and my older sister by one year), and went to an all boys high school, so most of my mates are guys.

Crossdressing:
I started crossdressing at a young age and still do it to this day. I put a lot of effort into it, I purchase girls clothing, makeup and wigs online, and I often go for walks in public while crossdressing (I actually own more girls clothes than I do guys clothes, and have a lot more skirts than my sister does). I love feeling girly and feminine, and I like it when I get catcalls and attention from guys while I'm crossdressing (it's very rare though). A lot of the time I feel jealous of girls because you can't do any of this as a guy.

Sex (as myself):
I only find myself sexually attracted to girls. I find myself constantly checking girls out, but I never check out guys. All of my sexual partners have been girls.

Sex (while crossdressing):
While I'm crossdressing it is a bit different. Crossdressing is still a very private thing for me (I haven't told any of my family or friends), so I've never had any sexual encounters this way. But the thought of me being a girl and having sex with either girls or guys turns me on. The thought of me being a guy and having sex with other guys does not turn me on though.

Masturbating:
When I masturbate, it is often while I'm crossdressed, and I usually masturbate to pictures of girls, lesbian porn, or porn where guys have sex with girls, never any gay porn. I have recently however found myself masturbating to pictures of some crossdressers or male-to-female transgender models.

I won't lie, I love being a guy and all, but sometimes I wish I had been born a girl just as much. I'd love to live life as a girl, I think that would be amazing. But I think If I were a girl I'd be a lesbian. So it's kind of like I'm a lesbian trapped in a guy's body if that makes any sense at all.

Anyway I'm really unsure about all of this, and its hard to come out as anything in the end when you don't know what it is at all!