Question about fwbs 22M

Since last august till early jan i used to have a fwb (F21) things were good and we said we will only sleep with eachother (both paranoid about stds) then on december she slept with guy but didnt tell me until early jan. After finding out i guess it ended because tht guy is very controlling however they arent in an official relationship. So my question is that should i try fix it as its normal i fwbs or me being upset is justified ? My main concern is stds and im sure they didnt use a condom. Another thing iv been wondering about is the fact we agreed that we will only sleep with each other and she did tell me i was the only guy she was talking to one day before the news about this other guy came up.

TL;DR: fwb had sex with a different guy n didnt let me know till way later and im wondering if its worth trying to patch things up