"I’m sick of having my parenting criticized"

My husband often wants to talk about how I’ve failed as a parent, how he gave me advice starting seven years ago and I didn’t follow it, how my 16-year-old son (his stepson) will never graduate from high school and be dependent on parents forever, etc. Husband keeps approaching this topic over and over again from different angles. He is never satisfied with my responses because, even though I’ve acknowledged my shortcomings and expressed that there’s some truth to his feedback, I don’t completely share his interpretation of the past. I am heartbroken because my 16-year-old son has moved out, and I don’t want to have to deal with my husband’s crap (his thoughts and feelings) about this on top of my own.

C Husband comments.
T I’m sick of having my parenting criticized
F Defensive
A argue, listen, tell husband I want to end conversation but don’t set boundary, validate his point of view, buffer, attack husband for his shortcomings, look for evidence that he is right, ruminate
R ??

I’m not sure where to go with this model because I’m not sure what result I want. I know I can’t change him. I guess I want to feel more neutral when this circumstance arises? But maybe I need to set a boundary that I won’t allow such conversations to go on for more than 30 minutes?

Thanks in advance for any help you can offer…