I am sorry you are hurting right now.
It sounds like you had wants and needs going unmet during his relationship with his Other.
And him breaking it off with his Other stirred things up in you?
Since you have decided to break it off with him as well, I hope you can break up smoothly, quick, and clean.
It doesn't need to be automatic dramatic/anger type break up. Sometimes break ups are quiet, firm, and loving because it is what is best for the relationship and the people in it.
I'll hope for a loving, firm break up then for you.