Holidays with hoarders

Hi :) I need an advice (obviously) I haven't been home for over 5 years (due to family issues). My parents are hoarding a lot of stuff and I just can't win this battle. Next week I gotta go and see them in their house. I'm planing to start cleaning the second I get in. This has been going on for over 20 years now. My partner of 10 years haven't been there yet and he's really insulted by this. I'm looking for a advice how to cope once I go there. Last time I went I got a mental breakdown. Honestly we were sitting on the table eating dinner and I started crying I couldn't hold it anymore. I need an advice please. How not go get in a year long fight with my parents and how to clean up and how to convince them to invite him at one point. I'm ashamed and I know they are too but please good people lend me your strength and experience on how to deal with this. Happy holidays and thanks in advance.