I knew pretty much as soon as it really started hitting me at age 14 that it had nothing to do with my sexuality, but it took me a LOT longer to figure out the full depths of it, along with what was attraction and what was dysphoria.
I was never attracted to men pre-transition, so that made any hypothetical confusion pretty clear. I asked myself many times "Am I gay?" and the answer was always no, so I knew it was something different from the start.
However, it did take me a while to realize that what I considered "attraction" to women wasn't the same as it was for everyone else. Like, at age 15ish, I assumed that every single AMAB person I was around was also looking at girls because they were jealous of them, and because they were imagining what it was like to be them. I NEVER knew that they weren't looking at women that way, that they were actually looking at them from a perspective of what they (with their actual bodies) wanted to do to them, or have done their own bodies by them.
It didn't matter a whole lot because my sex life was never something I enjoyed, mostly my experience with it is one of constantly trying to fight it, because it always felt wrong, and it made me objectify people who I HATED myself for objectifying. Plus there were so many aspects of gender that had nothing to do with sexuality which I was also jealous of, such as clothing options, social treatment, etc. So I knew pretty much right away that this wasn't just my sexuality talking, I really was wishing I was a girl.
I did worry that maybe my desire to transition was just a fetish, but going on HRT quickly remedied that worry, because everything else in my life that was an actual fetish (the things I used to hate myself for,) quickly disappeared right along with the testosterone. I couldn't get aroused by them anymore even if I wanted to. And yet the desire to transition stayed as strong as it had ever been.
I didn't realize the full depths of how my gender dysphoria and my sexuality have been interacting all these years, though, until about 6 months ago. I was at a point where I was trying to decide whether I wanted SRS or FFS first. And I'd always wanted SRS, but wanted to double-check my convictions, so I actually started doing some studies and experience-based tests to see if there was any way that I could ever be happy with my current anatomy if maybe I just found the right way to use it, or the right way to view it mentally. And that was when for the first time I started actually being aware that what I thought I was experiencing as "attraction" to women all these years wasn't even attraction at all, at least not what "attraction" means for every other person. I started studying what the common sexual fantasies are for cis men and cis women, and that was the first time that I realized that my experience was COMPLETELY unique. For cis men and cis women, their sexual fantasies occur in their actual bodies, or in idealized versions of those actual bodies. For me, I was NEVER myself in my own fantasies, I always had to transport myself into a female body before I felt anything. And yet disregarding that difference in gender, once I got into that body mentally, my fantasies were completely 100% normal... for a woman. And I also realized that basically all of my actual fantasies were of me in a female body being with a guy. I never had fantasies about being a guy with a girl, and rarely of being a girl with a girl. So that was when I finally, after all of those years of nothing but assumptions, finally learned the complete truth about both my gender identity and my sexuality... that I was a mostly-straight woman, and that SRS was going to remove a SIGNIFICANT hurdle that I didn't even know how much it was hindering my sex life until I started exploring it.
...After 16 years of being post-puberty, having been in an intimate relationship before, and having been on hormones for 2.5 years, FINALLY, after all that time, I knew. And I only learned because I decided to start exploring and comparing... asking serious questions like "what does a cis male feel? What does a cis woman feel? What do I feel? How did I feel when this was happening? How do cis men and women feel when these same things are happening? How therefore does all of this apply to me, and where do my experiences fit in to all of this?"