why is letting go so hard

My BF of two years cheated on me with a 20 year old he met at college. He's 32 I'm 30. He said he didn't want to lose me but if I chose to walk away then it was on me to give up. He wanted to stay with me but have the openness to continue seeing her. Like seriously...? He said I was old fashioned and how could I expect him to JUST LOVE ME. That he was too sexy for just one lady. I have two kids from a marriage. I feel like I'm at a point in my life where I'm past all that crazy fun. Been there done that. Obviously I had to let him go. It's hard moving on from this. I've lost 14 lbs in the three weeks that this happened. I loved him so much but couldn't share him especially when he was fucking that bitch with no condom. 💔