I heard even people who've undergone surgery and have been living happily for years sometimes doubt their "validity".
All I can say is to decide transitioning and regretting it is better than never attempting

you need to be more specific here. Are you doubting if YOU are trans? Are you doubting if transition is a good idea? or are you doubting your life during transition with others? The fact of the matter is transition itself is just a PATH, its not done tomorrow, it has no beginning, it has no end, it is just a path. So you go to your therapist and if that's all you need, great, if you need to go farther, then go farther. There's nothing saying "You are trans so you must have surgery and go all the way". So if you are doubting if YOU are, then that is something you need to discuss with your therapist, I personally have never had doubts with it. Whether transition is a good idea is again between you and your therapist and is extremely personal. the last being the reaction of others, this is why I have waited for 8 years to get to the last step, I have watched many deal with it, and more times than not those who tell their family "hey I am trans call me X" and the next morning are makeup dresses and heels are met with seriously stout resistance. Not because those around them are against it, but because humans resist change, and you just dropped a pretty huge change on them at once. Sure there are those against it, but that is why you have to decide what is more important, everyone else's happiness or yours. Is your life worth so much less than theirs that you are going to sacrifice your entire life of happiness just so they won't have some discomfort? I came to the realization early in life that there was absolutely nothing on this earth that could not be taken from me at any time including my own life. That's the world we live in, we lose jobs, we lose people, we get older, accidents occur, so on. So because of that, I had less worry about people turning their backs on me. Whether I transitioned or not, I would lose them eventually, so I was going to be happy while I was here and that was way more important then putting off the inevitable. I think in the end I had 2 friends drift away and that was it. but again I told them and waited for like 3 years before legally changing my name, another few years before changing my clothing around, so on, I let them adjust to change. So that one is all in how you handle it. What I can tell you is, once you are on hormones you will know if its the right course for you. One of two things is going to happen, your either going to freak out bad because your body wont like them, or its going to make the world a whole heck of a lot clearer.