I am so sorry for you. I understand your pain and would give you a hug - so for now a virtual hug just has to do.
Two things I’d like to point out.
You met him at a very young age — which is both a plus and minus. He took advantage of you b/c he could. Due to your age and not much dating experience, you trusted him and did everything a wife would. That is a marriage. Give and take. Have each other’s back.
But not having much dating experience you may not have seen the red flags. Maybe they were there. Maybe you were not aware of them early on but there may have been very subtle signs.
But please know he would have most likely done this to anyone he was married to. He sounds very selfish and manipulative. And that doesn’t happen overnight. It’s not a "you" thing / it’s a "him" thing.
It is shocking to a regular decent person that someone can be ok with seeing their kids on a very limited basis. Absolutely an awful excuse for a human being - and only a totally selfish jerk does something like that.
I’m sorry you are struggling with so much on your plate. Please continue to post here and get support. Find a good counselor for yourself if you can. There are books and podcasts out there for you. The Healing Library here at SI has wonderful resources for you.
Just know the older and wiser you would have seen through him. And run from him as fast as possible. He took advantage of you and as you said, groomed you from an early age. In your healing you need to see him for who and what he is.
And see yourself for who and what you are. A hero for your kids.