A therapist would help. Quiet reflection would also help. Think about if your feelings are only sexual in nature. If so, think about if you are really homosexual or bi. If you can't separate the thought of being a woman with a man, and you don't feel you want male to male sex, perhaps you are trans? Think about that. Also, think about if the need to be a woman during sex is more of a fetish. Do you crossdress? If so, is the need to be with a man a sort of fantasy, where the man is just a "prop" to help you experience the "fantasy" of a woman having sex. Also, think about if you ever want to be a woman, a regular woman, not a "hot" woman, or a "young, cute" woman, but just a regular woman, living a woman's life. Does that ring a bell? And finally, if you can get to a quiet place in your mind, try to think about if you feel your internal sense of self is female or male.
I know all of this sounds hard, and it may be a work in progress, but the exercise is worth doing to get a sense of self. It may be hard at first, do not dismiss any thoughts. For example, you may be jealous of young, cute women. That could be because you feel "robbed" because you are not one of them, or, it could be ultimately sexual in nature, related to a fetish.
People who want to be a woman during sex can do so for a couple of reasons - one, because they really are women and just haven't understood themselves, or two, it is a sexual thing. And, it could be a combination of the two. Many people have both situations. The key is to find out which is strongest. Did the sexual feelings lead you to feel like a woman thereby confusing you (not really trans but think you are), or is the woman inside peeking out? A sexual component is not a barrier to being transgender. But, i think that the sexual component should not be the driving force to changing sex. The only driving force for that should be an internal woman, if that is your real self.