I agree with Lovedove and Yoniandguy for that matter.
Posters attached her (?) for being harsh/offensive but actually she was getting to the crux of the issue and it's not something a bs wanting to continue their relationship wants to hear : that men who do this v often know their partner is going nowhere. They might minimise and lie to make sure they don't leave/try to throw him out, but they know she's very very highly invested, thinks she loves him, wants to keep her family together, puts lots and lots of value on the marriage etc.
This man knew op was going nowhere when he had his "mid life crisis" and chose to (yes, chose) respond to a distressing period with the disability diagnosis for their child by drinking, taking drugs, going "off the rails" az op put it while she held the fort; she wasn't happy with him but it sounds like there was no "sort yourself out or you're out, you're a husband, father, and family man, act like one (or I'll throw you out, you're making my life harder at a time of distress, you're not stepping up, you're flaking put etc)". He knew she was going to put up with it, persevere etc.
In a similar vein you have to suspect he thought she'd probably take the infidelity if she ever found out, if it was far enough down the line, and he minimised it. Blamed the diagnosis, blamed the drink and drugs, blamed their relationship state (his own doing), blamed the ow etc.
It sounds like, back then, he decided (whether due to mid life crisis or family crisis or both) that he didn't want to play family man anymore; he wanted to act like a young single man - drinking to excess, taking drugs, taking a sexual opportunity that came his way like he was single .... When he'd worked through that for a while he decided came back into the fold, knowing op would be there, holding everything together, committed, invested Etc.
Not really an equal relationship dynamic.