Because you are leaving the fate of your marriage up to a man who is having an affair and doesn't have your best interests at heart. By doing that you have no choice which makes you feel hopeless.
You have choices. I don't think being a doormat is one of them.
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.