You aren't overreacting, at all. My H had an EA, and I went through the same thing - things suddenly didn't add up, his phone was glued to him, etc. One night he forgot his phone, and in an instant my world crashed.
I too think a ONS would be easier than this. To see my H being so loving, so sweet, to another. Posting love songs, saying he didn't love me, etc. It hurt so incredibly bad.
My H had already ended it when I found out, but that didn't change my hurt, at all.
Affairs take 2 to 5 years to heal from, and that's with a remorseful spouse. Your H isn't remorseful. He's saying 'get over it', and he should not tell you how to heal. He decided how to cheat - you decide what you need to heal.
Have him read "How to help your spouse heal from an affair." It's short, to the point, and oh so correct.
In the meantime, try the 180. It's in the healing library (yellow box, upper left side of the screen). Go to faqs for BS, and then question 11.
You need to take care of you. Don't beg him to 'understand'. He won't. When he sees you detaching, he'll realize what he's done.
As far as her saying he's harassing her - that makes NO sense. Are you sure he isn't still talking to her? A LOT of people take affairs underground when they are discovered.
((((hugs)))) I'm sorry - I know the pain of an EA. It's very real.