I left ex when our twins were 18months.
He let me down BIG time when we came home from hospital after they were born and then disengaged from me and them.
I was unhappy but felt I had to stay for the sake of DTs
One Saturday I went to get my hair cut & do some shopping so I left DTs with him. I took longer than I'd said because I met a friend so came back 3.5 hours later to find both DTs in pooey nappies, obviously been like that for a while & uncomfortable and he hadn't fed them.
I realised that he didn't give a flying f**k about them and started making plans to leave. It took 4 months & in that time I got anew job nearer my family, sorted my finances, sold the house (which was in my name only - he didn't have a job), got a solicitor to draw up divorce papers and then moved into rented house near the new job.
It was hard & I kept it from him until everything was in place - he called me every name under the sun, made all the promises he'd made several times in the past & failed to honour again. His family (mother in particular) were vicious & nasty.
So my advice is make a decision, make a plan, sort as much as possible without your partner knowing, tell one or two key people who you can trust & who will support you then do it. Set your sights on the goal & don't waiver.