I am sorry I misread the point about counselling with your son. I still think looking at what happened and how it has affected you and your relationship with your son is a brave thing to do. And as you say, it is helping, so you should be proud of yourself, not beating yourself up.
Someone said to me - when I asked a version of why did I stay - she said to me where else would you have been? I had married in good faith, there were DC, I was doing my best to make things work. That is what we are expected to do. So where else would I have been?
The question is not how did you let it go on for so long, but why he did it for so long? (But you will never find an answer to that question, so it is a rabbit hole). You ‘let it go on for so long’ because it was your marriage, you believed in it and tried to make it work, surely, as we all did. I don’t think there is one lightbulb moment where you realise and end it, leaving is a process and of course, the perpetrator has put a lot of work in to making sure you don’t understand and that it is hard to leave.
And how you saw yourself as coping at the time, that is what helped you survive until you were able to leave.
And yes Squall the ‘it did not happen’; the perpetrator has also put a lot of effort into maintaining their sense of self as not an abuser. I went through a phase of thinking if ex acknowledged it, it would make it real and that would somehow be better - because he carries on like somehow everything was fine (whilst still crossing boundaries at every opportunity and pretending to be Mr Nice and Reasonable). It is still just a way of distorting my truth and gas-lighting. Whereas I know it happened and to an extent, is still happening. I have a very physical reaction of aversion at handover, because my body knows it happened and is still happening.
I was involved in a WA support group for a while and there were ladies there who came for support but were not going to leave, that was their choice. Maybe one day they will, but equally, marriage can be a tie which people feel strongly bound to, for all sorts of financial and social reasons, and those values can also be used to keep someone in a situation. The truth is that leaving is hard, and a massive step, and that is when the healing can start (aside from when you have to see the person and it gets a step back).
Anyway, I wish you well 