Well, since my teens my life has both gotten worse and better. It's been a rough ride, for sure. Especially during most of my 20's, I was a complete wreck. But into my late 20's I started feeling better and changed my perspective on life. Gender dysphoria is just one of all my issues I've been struggling with, so I knew all along that just transitioning wouldn't make me a happy person. I'd still be somewhere between miserable and fine-ish, just without/with less dysphoria and better self esteem. My transition was also (for the most part) so difficult for me to pwer through that had I known before it, I can't say I wouldn't have opted out of life instead. Even though I think it's been worth it now, I don't think my past self would agree.
The things I've come realise about myself and my past over the last few years, although encouraging to me now, would maybe not have been encouraging for my younger self to hear, but rather quite scary. Cause I have a completely different outlook on life now and value things differently.
But I know that if I did meet my younger self, he wouldn't take "sorry I've got nothing to tell you" for an answer, so to a me of any age between 15 and 19 (disclaimer sorry to bring some of my darkness to this positive thread, but I'm pretty sure my younger self would in some way have appreciated some harsh truth cause everyone back then kept telling me "don't worry, it'll be fine" when it clearly never got "fine" and that only skewed my expectations further. So these words would be empowering and just the right kind of kick back to reality that I did need back then) I'd probably say something like this in terms of my gender issues and transitioning:
"The best things in life never come without a struggle, and change will come after giving up, not before. And you'll give up many times on the way. I know you want for the world to change to your needs, but it's not going to. You only have the power to change your own perspective and learn how to slip through the cracks of the system. Keep practicing that deceitful mask, cause you'll need it, but keep treasuring who you really are underneath it too. Choose your battles and be ruthless when you need to. You're just as strong as you want to believe you are, that means you believe it too. You don't owe anyone your kindness but yourself, especially when people don't show you theirs. And for the love of everything you hold dear, shut up about Anna."
("Anna" is not a real person, but a fragmented part of my personality/my alter (due to childhood trauma, something similar to DID but undiagnosed) and transitioning most likely wouldn't have been possible for me had the doctors/therapists known about it/her.)