You are asking very good questions. You owe it to yourself, your husband (who you describe as a “great guy”) and your kids to slow way down. The affair let you pretend for a moment that you aren’t a mom of 4 with all that involves. But if you leave your marriage, you will still be a mom and you will then have to deal with 4 kids on your own, at least part of the time, and you’ll be mothering at least 3 of them who won’t understand why you would leave their great dad.
I strongly urge you to make an appointment with a therapist to help you sort this all out before you do something you will regret. It could be that your marriage has been drowned in taking care of 4 children, including 1 with special needs. It could be you have an unrealistic expectation of marriage after 11 years. It could be that the issue isn’t who you are with, but how you are with him. A therapist will help you figure out if the problem is in the relationship or in the way you and your husband have divided routines and tasks. There are other ways to get a break and reorganize your marriage without the shake up and emotional turmoil of a separation.
You might find this article helpful.
I wish you well.