I can't leave him because I'm terrified of what he'll do if I try to break up with him, the last time I tried to leave him he threatened to take my youngest from me and there wouldn't be anything I could do to stop him.
And how do you think your current partner would react if he found out you were cheating OP?

And how do you think your children will feel growing up in a situation where they know their parents hate each other?

Something needs to change, for your sanity and the sake of your kids. You are using this guy as a way to escape into fantasy to avoid your reality. Sort this stuff out before you run off into the sunset with another guy.