<human>: Can you condense the text into a few key points?

Was with my boyfriend for about a month (+several weeks prior intensely chatting online), everything was fantastic until one day we got into an argument and I broke it off. Then, realized I might have overreacted. He didn't respond to my email or text, and I thought I would not speak to him again so left it.

The very next day, I was asked out on a date and I ended up having a one night stand with the guy.

Fast forward until now (a week post breakup) and my boyfriend has contacted me out of the blue, citing "needed thinking time" and claims to have "seen the light", misses me, and has realized that he wants me back. He has apologized for his behavior and wants us to work. He has (for the first time) admitted that he loves me, and that he didn't realize this was the case until we broke up.

He doesn't know about the fling. And if I tell him, it will crush him. He has not dated or seen anyone else. 
I do miss him, and I also have strong feelings for him. I want to make it work. However, I feel like if I don't disclose this fling it will ruin things but if I do disclose it it will also ruin things.

Is it necessary to disclose this?
<bot>: Had a fling after a breakup, boyfriend wants to get back together but if I tell him about the fling it will break his heart. What to do?