Chuck - S3, E10 "Lantern"
I'm new enough to the subreddit that I can't be sure whether or not this has been covered from this angle.
Anyhow...Chuck's meltdown at the end of this episode (and its tragic climax):
I don't think his semi-forced retirement is the crux of this (nor, by extension, Jimmy's insurance sabotage or Chuck's falling out with Howard, which may have been a long time coming/death by 1000 cuts). You see his initial response to the end of his professional career as very responsible and proactive. He engages Dr. Lara Cruz, goes on medication, starts journaling, venturing out into the world, and really takes the bull by the horns in terms of soberly assessing the nature of his illness and working to find peace/resolution (perhaps a peace that he might not have sought out had he continued/been allowed to continue the HHM charade).
The fact that he is listening to music in a fully electrified house when Jimmy stops by to check on him (an unscheduled visit, so none of this is staged) feels like sufficient proof of tangible progress.
What DOES happen right before the wheels fall off so drastically is him rejecting Jimmy with a cold, bald-faced lie. We have watched Jimmy matter VERY much to him for three seasons (and in flashbacks, so...over a lifetime, really). Something about cutting his one living relative (and closest at that - the nature and extent of their relationship at the beginning of the series - and, again, in flashbacks...who is rooting more for Chuck than Jimmy in that Rebecca visit/cellphone/electricity charade? - is considerable) down/off/out/loose like that seems to absolutely destroy him.