When his behaviour started making the DC, especially DD, unhappy.
When I realised that I felt so much happier when he was away with work.
We didn't have sex for years because he'd lost interest and I realised that I dreaded the thought of him wanting sex again.
When I got covid for the first time, was very ill and frightened, and he left me with the 17 year old to go away for the weekend. Without even asking how I felt about it.
Things I ignored:
Him seeing a prostitute. 'Just to see what it's like' and 'just for a hand job'.
His constant criticism that I wasn't supportive enough even though I quit my job when he wanted to move abroad, was a SAHM doing everything for the DC and the home while he did the fun bits, pursued hobbies, etc.
Constant belittling, lack of caring, disinterest in me. No effort put into the relationship, apart from occasional bursts (what my therapist calls 'crumbs of attention').
It hasn't been easy and I still struggle, but overall I'm much lighter and less unhappy. Threads like this help remind me why this change is so important. DD has blossomed since we separated. That's been the best result of all.