When your other half starts acting shifty and disappearing a few days a week you get a little concerned. When that's followed up by sleeping in a bathroom one night and following that up by pretending to be on a call to your insurance company which later turns out to be a drug dealer...what exactly is an appropriate reaction? Ah, feck it, he's clearly gone mental but it's his life so I'll leave him to it?
After listening to a series of non-stop lies for nearly a year with the tip of the iceberg being a fake fugue state you get to the point where there's a complete dissolution of trust and honesty, so separation seems a reasonable next move. And of course the thought of your partner being a crystal meth dealer makes you worried about what kind of danger that can lead to. When your partner decides to move back in and then makes you seem like an idiot, playing the "perfect parent" card and generally acting like that whole crystal meth double life doesn't really exist...well, you would go a bit mental. You're helpless because any decision you make will destroy your family, so you inevitably cave in and let your partner "win" the battle to minimise the damage done to the family. Resentment overload, time to lash out, you know this is the one thing that will drive your partner crazy...how exactly does cheating on your partner override all of that and make you into the bad guy? Bizarre.