If you are truly trans, you will come to know (and it may take time) that it is definitely not a phase. Transition, though, isn't something to jump into if you're not sure. Just because we're doing it, doesn't mean you have to. You need to do things on your own timetable. If that means hashing stuff out with a good gender therapist until you're sure one way or another, so be it. Just don't let family and friends push their own doubts and issues onto you.
Transition because you NEED to, not because you're supposed to (per some unwritten trans rule somewhere).
At my age I knew it was the right thing, and figured even if I was wrong, I'd already lived half my life as the wrong gender, so if transition was the wrong thing, living the rest of my life as the wrong gender wouldn't be any different. 15 mos into transition and no regrets. Did it make my life 100% perfect? No. But it got rid of the daily angst and I can live as me.
Jay