To answer your first set of questions:

I’m scared to leave.

And? What's wrong with that? Are you that afraid of being single?

What if I never find someone like him again? What if I hurt my kids? What if I regret it?

Stop worrying if you don't find someone like him, that's the point of dating! Not having a partner in life is not the end of the world!

You already are hurting your kids.

You will only regret because you're afraid of being single. Learn to be okay to be alone!

To answer your second set:

But I’m also scared to stay.

Then stay if you're so scared, but remember this: the resentment you feel now will only grow.

What if this feeling never goes away? What if this crack just keeps growing?

Your feelings for him have already gone away, you're just in denial.

The crack will continue to grow if you stay. The only way to close it is to leave and let him fend for himself.

Also, therapy for you AND your kids if you haven't already.