As someone with counseling experience - I think you may want to tell her how you FEEL - EXACTLY. And tell her you want the letter, it is what you KNOW you want, You FEEL she is wasting your time. And tell her that if she does not have enough experience to write a letter, she could just be uncomfortable with this type of thing, the only ethical thing for her to do is to refer you to another therapist who can write the letter. I really think being direct is going to help. Also, ask her how her "plan" has anything to do with what you are seeing her for. Many people don't have 5 year plans! Should everyone have a road map for life - yes, it would be a good idea. BUT that is not what you are seeking treatment for. You are not there for career counseling, or learning skills to help you reach your goals. All very reasonable and legit reasons to seek counseling. But if you know what you want, and you are sure of it - tell her.
Additionally, while I am about to start to see one myself - I've done a bit of research here and on other boards, the point of therapy is not, and I repeat not to tell if you are Trans - as another poster stated earlier, no one can tell but you. It is to make sure that you do not have any other 'underlying pathology that could be causing you to feel like you want to change genders, i.e, transitioning to please a parent or something.
Good luck.