You're not alone. I went through the same thing when I realized that my dysphoria was bad enough to warrant seriously considering a physical transition. The cycles of doubt still ebb and flow, just far less rapidly these days (about a year in now, HRT since summer last year.)
I firmly believe that doubt is good. That's too superficial an explanation and verging on a platitude, so let me fill in the details. I believe that pre-decision doubt is a very useful tool to ensure that we're asking ourselves the necessary questions about these major life-altering changes, and I believe once a decision has been reached and a course plotted, doubt serves a post-decision role as a means by which we can check (once in a while - not all the time) that we're still on the right path. It's good to second-guess yourself.
So go with the doubt. It means you're thoughtful and you want to get things right, and it in no way indicates that you're not transgender. You're (rightly) wary of leaping head first into what often ends up being, on a day-to-day practical level, a social, professional, financial and emotional storm. Embrace your doubts and resolve each one as and when you're able. You don't have to make a decision on anything today, tomorrow, or even next year; you've got plenty of time to iron these things out. But the flip side to embracing doubt is that you can't let it prevent you from exploring who you are. If we waited until we were 100% sure about everything we did, we'd never get anything done. Take baby steps towards transitioning in the interim - and I note that you are, therapy being a great place to start. We don't all begin with the biggies (e.g. "Mom, Dad, surprise! I'm transgender and I'm gonna be a girl!"), and it's fine to take little steps and figure things out as we go along. Often the little steps give us the confidence to take the bigger steps.
I guess this is a roundabout way of reassuring you that it's okay to proceed with caution.
