Realising that I was more unhappy than happy in my relationship, and it was all down to his abusive passive-aggressive behaviour.
The short version is; about 6 months into the relationship it was made very clear that I was at the bottom of his priority list. I'm not a diva-demanding-attention type, but it was all the broken promises that finally ended it.
e.g. On many occasions of our pre-arranged dates, he would just not... be there. He'd leave me waiting outside his house for hours (literally; the longest I lasted was nearly 2 hours until I realised how pathetic I was being) at a time, not answering his phone and no text to apologise or explain. He would unexpectedly ditch me at the last minute for work - he didn't need the money, and this happened a couple of times a month. He would promise not to do it again, and then do it again the next week.
I tried for months but he was unwilling/unable to listen why I was so upset at constantly being treated like an inconvenience to his life, and he would turn it around on me all the time ("your life is already sorted out... I'm trying to make my life better, why can't you be supportive... why are you trying to make me feel guilty about that..." etc).
When I asked why he was telling me everything I did wrong instead of explaining/apologising, I never got a straight answer, but it was definitely all my fault. As I told him when he came crawling back (twice!) I've got far too much self-respect for all that nonsense. You had multiple chances, now jog on love.