Just a heads up...this line of behavior aligns with my experience with people dealing with substance abuse. You only mentioned that they were "getting high" but not what substances they were dealing with. Knowing no other info, I would probably guess they were using something beyond marijuana. The irrational nature of him hanging around with this other woman when things were seemingly going great reeks of guilt driven by the need for more convenient drugs.

I say this as it might bring you a little closure. He might not have always been as much of a trash can and something like drug use could have exacerbated some mental illness or character flaws beyond repair.

Either that or he just fuckin sucks. 

Either way, fuck you Tom.