I think he needs to go to counselling on his own. And you should also go to counselling on your own to figure out your needs for your life and what you want.
You both have a dynamic going on in this relationship. All relationships are a dynamic but some dynamics include a lot of issues from a person's past, which only make themselves known as the relationship develops over the years.
It sounds like he has a compulsive need to get attention for himself from females. It's like an addiction to getting attention.
He uses you as the secure base, you are the substitute 'Mummy', then goes off to play with other women on his phone.
If one was to psychoanalyze it, he has a Mother problem.
He harbours both anger towards his mother for her belittlement of him, plus at the same time a desperate need for her attention.
His need to hurt his mother's feelings will be expressed by messing you about (you are the mother figure because you have shown you will stay no matter how bad his behaviour ).
Yet at the same time his cravings for attention remain.
Because being with you (or any steady relationship) doesn't solve those deep rooted issues for him.
If you had kids things would get worse, as your attention would be diverted towards the kids.
I would guess if you threaten to leave him, you will see him turn in a sobbing little boy crying for his Mummy. But that still won't be relating to you as an equal adult. It'll be all about him being upset, I'm such a bad person etc etc.
There may still be no 'adult' perspective on how he has hurt you or consideration of how you might feel or the implications for your shared future.
What you would be looking for in counselling is an adult perspective from him. But he'd have to be seeking those changes himself, for his own sake because he feels he'd like to feel different about himself, and I'm not sure he is?
You are allowing him to talk to you like this. To threaten you by saying these things. I get the feeling you don't let him know the full impact of the things he says to you about your future. This is your hopes and dreams he's fucking with, he's toying with.
Writing a long letter is not a strong emotional reaction in the moment. It's quite a passive, considered reaction. I wonder how easy you find it to express your own emotions to others.
But that's all Him, him, him. What about you? How do you feel? What do you want from life, what treatment do you deserve from an equal partner?