Few options really:
Either the trans girl gets lucky, they find a partner or a friend who recognizes whats going on, and gives them a safe outlet to start exploring who they are and they realise that they don't just want to pretend to be a woman sexually, but that they're actually just happier being a woman.
Second possibility is the trans girl has a moment of clarity on her own, possibly late night by reading an article, going to a trans subreddit like r/asktransgender or find a trans post (such as this one), that gives them permission to be a trans woman, to transition and become a woman, live life as a woman (you totally are allowed to be one if that's what you want).. And this manages to catch them in a moment that overcomes their repression, their inward facing transphobia, and fears of what family/friends etc will think.
Third option is kind of the meta reverse lying to yourself option: Where the trans girl says "Ah I'm not trans, but ill pretend to be one, so that I get to transition. haha lol. That'll teach them. I'll take the HRT, grow nice tits, do the exercise, have a great ass, see the gender psychologists, pass the tests, train my voice, have the surgeries, but im not trans, I'm still a man lol." They then figure it within the year when they meet other trans women.
Final option is that their gender dysphoria (that most trans people are unaware of until they come out and get past their repression) grows so bad from their ageing male body, and the resulting knock on mental health issues it causes and the trans girls coping mechanisms to cover start to manifest in a way that its kind of sink or swim. Possibly they literally break down and see a therapist who if they're lucky will be good on gender and help them.
Common issues: Drug/Alcohol Abuse, Self Harm, Dissociation / Depersonalization (lack of emotions, distance from yourself, feeling of constant brain fog), Increasing risky sexual behaviour.
The main trouble a lot of trans people stuck in this repression have, is they want to be "real" women, and not trans women (which in their mind is not a real woman, just a man who has decided to transition). They feel that there's not enough to be gained from transition, because if they can't be a real woman / super fucking porn star hot, then its not worth the effort.
No one wants to be just a man in a dress, everyone wants to be a woman. But really the desire to crossdress will never go away unless you transition. No matter how old you get. So by not transitioning, thats how you make sure you'll always only be a man in a dress.
So people reading this, if you think you might actually want to be a woman; Have the tits, get the amazing juicy ass, the round soft hips, wear the cute clothes, be the girl, fuck the boys boys boys (or whoever you want), live the dream, your dream (the one where you wake up a woman everyday and stare down at your soft hot curvy body and smile (as I am right now)..
Then yes, you're allowed to be trans, you're allowed to transition, you're allowed to be the woman.
Get after it girl.