I am sure you will share more later, but in the meantime, take things very, very, very slowly. It is OK to say things like 'I need to think about this' or 'I need some time, can we discuss this tomorrow afternoon?' or 'this is a big shift for me, I need to consider all my options before I feel comfortable responding to you'.
You DO NOT need to take him back right away. In fact, if you do take him back right away, it diminishes your value to him and when OW comes crawling back in a week, you are back where you started.
You are not Plan B. Repeat that to yourself until it is a second layer of skin. Don't allow H to relegate you to Plan B.