My boyfriend cheated on me with a 40 year old woman?

I'm 20 and he's 27, we've been dating for 3 years. Sex was great and everything else as well, but last week he confessed he cheated on me with one of his collegues. I feel betrayed and I can't understand why he did. Our realtionship was amazing and I loved him a lot. He broke my heart and he cheated on me with that old woman. I honestly wouldn't feel this bad if he cheated on me with someone hotter, I could get it, but like this I cannot tell why he would have done that.
My boyfriend cheated on me with a 40 year old woman?
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