The only way I'd ever forgive a cheater in a serious relationship is if it was a mistake that won't happen again and the relationship was amazing.
If its a months long affair GTFO
if it was numerous times or multiple people GTFO
if she doesn't intend to change the offending behavior (getting too drunk/high without you, partying without you, too many orbiters/attention whoring, etc.) GTFO
if the relationship was less than great or she is less than satisfied with you GTFO.
If the relationship is less about kids/marriage/partnership/cohabitation and more about short to mid term fun then you can use it as leverage to transition into an open relationship or downgrade to FWBs/plate status or something.