do I have the right to be mad or should I let it go?

So long story short a conversation got brought up between me and my bf about the last time we broke up which was last year. So Literally the next day after we broke up he had sex with another girl. Considering what we had wasn't a 2 week relationship it was 4 years during the time so I was completely devastated at the fact he would mess with someone other then me within a day but I only found out that happen because he told me...he said he was sorry and wanted me back and cried to me ..I asked what the girls name was and he said he didn't know during the time which I don't get how he didn't bc he told me he took her to the movies before hand too then he fucked her . He just kept  saying "i don't remember and I didn't care I just wanted to make u mad and i messed up" Well anyways it got brought up today and it somehow got mentioned that he does no the girls name so I'm thinking ok it was obviously someone I would be mad about or someone I know or someone I hangout with !! so I feel like I have the right to no who it was bc now I'm looking at every girl that comes around like "it was probably her" it's eating me alive . Why doesn't he want me to no who it was ?? Is he saving this girl and himself by leaving me in the dark?? Do I have the right to be mad that he won't tell me who it is?? Even if it was a year ago?? Ugh still makes me mad what should I do ?? What would u do?