The tipping point was pretty much when I realised my life would be better and I could transition.
I was very confused and I had no idea "who I was" for a long time, sometimes even know I wish I had a clearer picture of my mental state when I was younger as a form of reinforcement and validation of my choice to transition. Since I hear so many stories of suicidal gender dysphoria and knowing from a young age but I know not everyones the same.
I've been crossdressing and having fantasies about being a woman for a few years, but never thought anything of it. I hadn't experienced any suicidal dysphoria, really ever, but at the same time, I can't say that I was "truely happy" either. When I was younger, I was extremely depressed to the point of suicide, but I had no idea of who I was then. And even after that when I got better (mainly because I became religious), I rarely smiled, I rarely made jokes, I frowned a lot and everything in life was hard but I just pushed on and thought, "I guess this is normal" and it was certainly a hell of a lot better than the depression I had suffered when I was 14-17.
As I got older, I went to university, got into a long term relationship with a woman, that lasted about 6 years and through that time my desire to be feminine just grew, I had no idea why but it became more intense to the point I was dressing and living part-time as a female whenever I could. Then 2 years ago when the relationship ended (partly due to gender issues), I tried to imagine myself as a male going through another relationship, working as a male, living life as a male and just saw no joy or future in any of that and was depressed for a while. I just put that off as "post breakup blues" but now that I think back that was my first case of gender dysphoria because I learnt about a year after that, imagining a life as a female, brought me immense amount of happiness. The more I thought about it, the more I realised this is how I wanted to live my life.
The real tipping point came about 7 or 8 months ago, when I realised that I could do it even though back then I just thought about going stealth and hiding it from friends and family, that gave me the kick I needed since that was my biggest fear. I have like 2 friends (1 which knows and is supportive) and am not very close to my family, I have very little to lose and know that I'm going to be happier, I decided that I should come out to them, even if it doesn't work out for whatever.
I think if I hadn't been religious or hadn't been in a relationship that I clung to for emotional support, I would of been dysphoric a lot earlier and a lot more intensely. I'm still dealing with all the religious and emotional baggage that I put on myself to avoid facing reality but when I look in the mirror, I see a female, it feels right and that's good enough for me.