If I'm honest, mine was probably a year into our relationship, though I didn't see it at time because I didn't know any different - first 'proper' boyfriend.
Mine was on Christmas Day, over roast potatoes of all things!! Another story...
Anyway, like you it had been bubbling away in the background for years, my children were at secondary school and the eldest was in the middle of GCSE's - what the fuck was I thinking 
He thought I was joking at first, played the woe is me card, all my fault, blah blah blah, but my mind was made up.
Scary as hell, being on my own after 20 years with him, but that was better than what I was stuck in.
DS's survived! Eldest now in second year at Uni and youngest likely to go this September.
I rented, not the best house, because I couldn't afford a mortgage. Not that I cared to be honest.
I cannot underestimate the difference it's made to my life. Yes I had the worries of 'how will I cope', the bills etc are all down to me.
I had always played the mum and dad role, he wasn't an 'ideal' father, and you know what? Nothing changed! Me and DS's carried on as normal, conversation very open and they didn't fell either one of them had to 'dad' in the house.
Life is too bloody short for regrets. My only regret is that I'd have split sooner but I'm out the other side and that's what matters.