Several
The first - having one of those gaslighting, interminable arguments with abusive boyfriend. It was about a fairly random political issue but it was something I had studied, written on and was well informed about. He knew nothing on the subject and had no personal skin in the game. So I was holding my ground calmly with the facts rather than becoming conciliatory and emollient to hold the peace between us, thinking we could have a rational discussion like adults. And he got really riled up, practically frothing at the mouth, jumping about the room, and then started accusing me of being overly emotional and shouting. I knew nothing about the concept of DARVO at that point but suddenly realized this was how all out discussions and arguments went. He wasn't arguing and discussing because he had firmly held opinions or investment in the issue, he was very much playing the man and not the ball. The situation was engineered so that he would 'win' by provoking me enough that I would either get upset or falsely concede to his side in order to hold the peace, and then he could paint me a stupid, irrational, emotional, weak, needing his informed opinion. It was honestly like the scales dropped from my eyes at that moment, and I became utterly icily calm as I saw the whole. And so continued calmly holding my well informed ground and didn't get emotional when he tried to make it personal. He stormed off in the end as his usual game was thwarted. Id love to say I ended it that day, but it took a few more months to get disentangled, but that moment was the revelation and understanding of the emotional abuse and manipulation tactics he was using.
The next in my 20s - had just been offered a prestigious post in a particular research center I was working at, whole career path opening up and mapped out for me. Was sitting in the canteen one morning, looked around at everyone and realized this wasn't a future I wanted, went straight home after work that day and applied for a new undergraduate course that would totally change my direction, much harder path to carry out and fund, but the right use of my life and talents.
30s- a particular incident with DH where I realized we would marry and have children together
Again 30s - failed in a job interview, and was really upset. Went to bed after a glass of wine and bit of a cry/sulk. Woke up at 4 that morning with what can only be described as a feeling of absolute peace, calmness and joy. Lay in the dark thinking about it and realized that I was happy that I hadn't got the job because that meant I was still free to make plans, didn't have to commit to that particular workplace and colleagues. This was the impetus to make changes that have led to working abroad, learning a new language and a much better life and job.
I don't make decisions by waiting for a gut feeling - but when my gut says something I listen to it as these 4 epiphany moments have definitely shaped my life for the better