It seems to me that if they aren't actively against the idea of you transitioning, but just aren't sure you're serious, you need to force the topic with them: without beating around the bush, you should just bring up the subject, and try to discuss it in a meaningful and productive way with them. I think that being indirect has a time and place in this process, but when I was in your shoes, my folks first thought was that I wasn't mature enough to know what I was for certain, and the best way to convince them otherwise is to be direct, sincere, and honest. I know where you've been, and things can get a lot better! I hope everything works out for you!