If you're like me, then your feelings of asexuality are actually due to environment/circumstance, but you already seemed to have accepted this as a possibility.
In my case I've felt straight my entire life, but the amount of physical attraction I've felt towards females has always been extremely low; for the longest time I chalked it up to me being asexual, with a hint of heterosexuality (sounds completely ridiculous, right? XD), but this wasn't because I honestly felt this way, it was, more likely, because I had never been in a romantic/sexual relationship before.
Only recently have I come to accept that a large part of this was me suppressing my own sexuality so that I felt like I would never want or need a relationship, not that I have to be in one now, but I'm able to accept that it's not something I entirely loathe (it IS nice to hold someone close to you...), that a relationship does have its own positive points. As to my physical attraction for the opposite sex... it's definitely there, but I've found that having compatible personalities plays an absolutely crucial role, as does accepting the person that I'm in a relationship with as a woman.
But more about accepting someone as a woman... I think my attraction to someone depends on their gender instead of their sex. I don't believe I would feel any more attraction towards a woman who associated themselves as a man, than I would towards any biologically born male walking down the street. In the same way, I know that, if someone associates themselves as female, I can feel attracted to them, regardless of which sex their body currently is.
I guess that means I'm straight, but in the sense that I'm attracted to the female gender. At least, this is what I know about myself so far... I'm sure there's much more that I will learn about myself in the future.
As a final reply to your post (I like to go off on random tangents I guess...) I don't think you'll ever be able to make yourself turn asexual if you aren't already. It's probably healthiest to accept yourself as being straight if that's how you truly feel, but to not let your attraction to the opposite sex haunt or depress you because of your environment or circumstance.