The only time I doubted was right before I came out to everyone, and it wasn't because I was afraid to transition, but because I was afraid of others' reactions. Turns out I didn't lose a single family member that I cared about and I didn't lose a single friend either (in fact, I gained two new best friends and strengthened my relationships with my old ones).
If I had to go back to living as that boy, I don't know if there's a hospital in the world that could keep me alive. My final choice to transition literally came down to whether I was going to let anorexia kill me (5'11" and 100 pounds at the time) or become the woman I knew I was. That was almost exactly two years ago, and now that I've been full time for four months, gained 40 pounds, and am healthy and happy, I would never give this up and I would never doubt. I already have an entire lifetime of doubts to contend with, the last thing I need is to continue doubting the very core of my physical being.
When I first started going out in public, there was fear, but no hesitation that I had made the right choice. Since hormones are sorting my body out, I've done nothing but gain confidence and strength, so why would I want to be the timid boy ever again? There are far too many moments where I'm walking around in a tanktop, short shorts, cute sandals, hair tied up, and I'm with friends and laughing as I've never laughed in my entire life. I feel, I love, I care, I live a life that I thought only other people experienced. Before transition, I was hollow and miserable, and nothing got through to me. There's no other way of putting it: transition saved my life. That's something I will never doubt.