It is a matter of some serious curiosity to me that some people are emotionally triggered so intensely by the thought of their lover also having another lover -- which is a different issue, altogether, from the matter of deceit and hiding of "an affair". Part of my curiosity relates to the fact that I'm not entirely sure how, in every detail, I transitioned to the "place" I am with regard to this matter. What I am sure of is that if my partner, Kevin, loved (and made love with) someone else as well as me, that would be fine--so long as he took appropriate precautions vis-à-vis STDs, was honest and open with me about that relationship, and didn't retreat or neglect my need to share quality time with him.
It wasn't always the case with me. I used to be situated on the monogamy end of the spectrum pretty solidly. I think that was mainly because I bought into the pervasive belief/attitude that love ('romantic' love) is only love, really, when it's expressed in a dyad--and anything "less" would "cheapen" it or ruin it. I suppose I got that from the culture (parents, 'society', Hollywood...).
Anyway, you've got SO MANY issues to examine and re-examine all at once. I think it is best to deal with them individually, rather than let them all be a mush of interconnected confusion. They are all intertwined in some way, but can be teased apart enough to be examined individually. First, you need to decide whether you are willing to forgive and move on about the matter of your having been decieved, or "cheated on," as they say. It really is cheating when there is hiding or deception, so you have every right to feel hurt about that, but you have to decide if you are willing to forgive -- and also whether you are using your justified hurt (read: anger, resentment) as a manipulative ploy. I'm not saying you are, but you need to look at that carefully and ask yourself whether that's happening and how you feel about it if you are. Just because anger or resentment are justified is no good reason to harbor them. It isn't good for you and it isn't good for your relationship to harbor resentment and anger.
Also, look to see if there isn't any past experience (wound) being triggered here -- one predating this present experience. If there is such, this may be contributing to the intensity of your hurt, anger, fear, resentment.... By seeing that the present experience and the past experience are distinct from one another you may begin to discover how you can better deal with these feelings.