So I'm forced to be a person I'm not.
You're absolutely right on this point. You were forced into being someone you're not because of the affair. It churn up all sorts of emotions that you would typically have under control.
There was one time my son forgot his pumpkin for a class project in the 1st grade (he's in 4th now) and I had to go to school to drop it off. The secretary asked me how I'm doing. Simple enough question, right? Well, I burst into tears and all I could say was "my son forgot his pumpkin." and I just about ran across the parking lot to get back to me car. I felt so
I think being nice to him would make him think everything is ok
I've had this thought as well. I'm 5 yrs out and I still have this thought. What he did is not ok and I feel he needs to see you like this so he can see the depth of the pain you feel. It can't be brushed off as "no big deal."
How does he react when he sees you like this?
It very well could be that you put your guard up. You don't want to be hurt any more. In time your emotions will settle back down and you'll be the person you were before, but now you'll be stronger version of yourself.