Your reaction to your boyfriend sounds quite could and callus. I wonder how you would feel if the tables were turned and he cheated on you with your friend and room mate? How would you feel if he told you to just get over it?
Neither of you should be trying to forget about it. The fact that your boyfriend keeps bringing it up means that he is traumatized. Whenever we return to an upsetting topic we do so to try to work through our feelings. We also return to a topic again and again because we haven't received the response that we need from the person we're talking to.
imagine your boyfriend feels done wrong by you, and judging by the unfeeling tone of your letter, it doesn't sound like you are understanding his pain at all. Therefore, he's not hearing what he needs to hear from you that would enable him to move on.
Maybe you are finding it hard to respond to him because you feel guilty to be accused. Guilt leads a person to feel angry. I think you just want to escape these unpleasant feelings and this would explain your demanding that he just drop the whole subject.
Rather than dropping the subject, you both should be trying to understand why the cheating happened. What is wrong in this relationship that made you feel the need to cheat on him? What was your boyfriend doing to push you away?
When you both understand why this happened and you BOTH makes steps to rectify the problems in your relationship, then.
Your boyfriend sounds like he might have a tendency to play the victim. He may have unconsciously set up a scenario where he would'help' you to cheat on him so that then he could complain about how you did him wrong.
People who play the victim are actually releasing mountains of anger without anyone knowing it (including, they themselves). You see, the victim is saying without directly saying it, 'Look at how you wronged me you bastard, or bitch. Look at how I suffer.' You can see that there's a lot of anger being released.
Obviously, the person who plays the victim isn't just releasing anger from the here and now. He/she is also releasing all the buried anger that 's trapped inside since childhood. The problem is that victims will set up scenarios in which they arrange to get dumped on, so they can release part of the anger that 's trapped inside.
Unfortunately, this plan never works because when the victim arranges to get dumped on he/she is actually creating more and more anger inside him or herself and adds to the festering pot of anger rather than getting to the bottom of it.
It would be good for your boyfriend to look at what he might be doing, without realizing it, to encourage you to cheat on him, so that he can play the victim.
As for you, you sound very angry to me. You need to look at whether you are punishing and torturing your boyfriend in order to get even with somebody from your early life (mom or dad?) who did you wrong.
I have given you both a lot to work with. It won't be easy to face yourselves, but if you want to have a relationship that works, you both have no choice but to investigate these dark corners of your selves.
If you find what I suspect is lurking, own how you feel toward the people who mistreated you way back when, work through the anger and hatred. This is the only way for you to immunize your relationship from future infidelities.