We never truly stop second guessing ourselves. Our personality and self is a fluid idea, and we are always shaping into new beings. If it is in the regard of gender, however, discuss it with someone close to you, or one of our listeners. Seeing it on paper or hearing it out loud when telling someone else could be the reassuring factor you need.
You are the only person ENTITLED to guess and second guess and wonder about and take time to figure out your identity. All aspects of it. If it's sexual identity, that can vary through the lifespan -- who you're attracted to, etc. If it's gender identity, asserting yourself as transgender is a big step that deserves a lot of support from family (ideally) as well as friends, mentors, support groups, other trans people. 94-95% of people who've transitioned reported (in a study) that they had no wish to transition back. But that journey might take you a while, so surround yourself with people who see you as you truly are and make sure to make that only a part of a full life (school/work/exercise/creativity/social/contribution to others/appreciating beauty/time outdoors...this will help you know all the layers and colors of who you are.
Don't second guess! You are who you are, and all you need to do is explore it. Sure, you may change, or react to a situation in a way that's unexpected to even yourself,but it's all a part of your identity. Don't think of your identity as something you need to control, but more as something you are and something you grow into.
There's nothing wrong with second-guessing. It is your identity after all, and if you feel like it changes from what you thought before, it is entirely your choice to move on to the new identity. Sometimes it helps to give specific traits to your identities when they change back and forth, that way you can better anticipate how you'd feel.
There are two ways: don't worry about labels and preconceived notions of what sexuality is: after all sexuality is a spectrum and labelling it can imply otherwise, or there is probably a whole community full of people who have the same feelings as you and are labelled the,selves as they see fit, it is just a case of looking.
It may be helpful to read ideas about gender/sexual fluidity. The most easily accepted form of sexual fluidity is the way ones sexual feelings and desires (or lack thereof) change across their lifespan from birth to death. More LGBT+ friendly ideas explore how one's feelings can change over years, months, days, or even hours depending on the person. It can help to star exploring this idea in general, and then seeing if perhaps that might help you be more at peace with fluxuations or ambiguities in your own identity. It is not an idea you have to accept, but exploring the topic may help you in this time regardless of your final feelings about the idea. It can also help yo give yourself time and say, "I'm not sure now, but it's okay. Maybe I will be sure in the future. I can give myself time to grow and change." And, of course, any positive and supportive interactions you can have in real life and online may help support you on this internal, personal journey to self-acceptance. I wish you the best. :)
I have yet to find a 100% perfect answer for this because I struggle with this as well. I can assure you sometimes you'll ask yourself "Am I just making this up?", but once you test some things out and get a feel for what you're after, you should be pretty confident in yourself and not second guess so much :)
To stop second-guessing yourself, you truly just have to get in touch with yourself, but you have to understand you will always be changing! Your life is never set in stone :) It really depends on the person and how far along you are in your life!
It's something I still have problems with, it's hard to always be sure so I always so go with how you feel at that point, there's no shame in changing depending on how you feel until you know what's right for you