I too am a FTM with ADHD, and I get your doubts. I went about it the logical way. I researched, read, then researched some more. For well over a year. I read everything I could find about what going from AFAB (Assigned Female at Birth) to male would do, doubly more since I'm well into menopause. I knew that it would do all kinds of changes to my body, and yet it felt like the right thing for me to (finally!) do. I'm going to say this here and now,
do your research! I'm going to try to answer your questions/queries in the order that you posed them.
Remember how you felt when yo wore men's underwear, packed or bound for the first time? How confident you felt? That in itself should tell you that it's more than likely the right thing for you. And the first time someone referred to you by your true gender? Same thing, if it felt right, felt good, felt empowering, it was probably the right choice to experiment with the
idea of transitioning. If you still feel that way whenever these things happen, then its probably worth it to you to keep doing them. Growing leg hair, and everything else, does that feel right to you? Then maybe you should continue to do it.
Like it was mentioned before by
KathyLauren, change is scary! Its okay to be afraid of what could happen.
If you don't feel good about being called he/him/his, then tell people to call you something else. Besides "he/him/his," there's always they/them/theirs, and even ze/zim. Here's an article that explains further:
https://www.mypronouns.org/ze-hir/ Try them all and the one that makes you feel happy is probably the correct one for you.
-If later on down the road, you realize that you're not trans, simply stop. Stop hormones, don't do any surgeries that you may have planned, just stop. There is no "law" that says you must have everything done. Plenty of people stop their transition, or even detransition. Of course, there will be things that are irreversible, like vocal changes and bottom growth, but there are also many that are reversible. Remember what I said about "do your research?" Here's more of it.
http://www.ftmguide.org/ttherapybasics.html Chances are, if you have any doubt about your gender, you're somewhere on the transgender spectrum. Even if you don't do anything but say, "I'm trans," then you're trans. Many transgender men still dress feminine, or do things that are considered by our society to be "feminine." So what? If it makes you happy, then keep doing it! Wear that lipstick, nail polish or hair bow! Its not hurting anyone.
I misgendered myself quite often until relatively recently, After so many years of thinking of myself as 'she/her," it took me a while to change that mindset. I even still heard my old, higher-pitched voice in my head when I thought! But now, after a bit more than two years on testosterone, I can't even remember what my old voice sounds like! I'd have to go find old videos of me and listen to them to remember it.
And if none of this is you, then there is still the option of being non-binary, androgynous, neutrois, or even different levels of thinking of yourself as male, such as transman, transguy, demiboy, or many more that I can't think of right now.
Experiment. Try different names, different genders, different ways of expressing yourself. And if people don't get it, don't worry.
You may not be sure of yourself yet! It happens. Not everyone that is transgender understands what their needs, gender expressions or even their
name is right out of the gate!
The bottom line is this.
As long as it makes you happy, (and you're not hurting yourself) then its okayHave fun finding out who you are to be happy! Enjoy yourself and welcome to the forum!

Oh, one more thing to add to your research. This video series might help you sort out your feelings.
I recommend it to everyone, since it not only explains things, but is very informative.
Good luck!
Ryuichi