I've had several downswings over the years. Some of them big ones like yours.
Number one reason will be tilt, even though some people will deny it and say they are playing reasonably or even ok.
I have two advices.
First one is quit for some time. Go do something else, forget poker. This one will be difficult because you'll want to return to the tables to get your money back, but give your best effort.
Second one is to drastically reduce your volume and shorten your sessions. Set up the playing time before starting to play and quit no matter what. You'll also have a better time when reviewing 1h sessions rather than 8h sessions.
Following up on the previous one, a most general advice is to play every single hand taking into account every single factor you can think of, it will improve dramatically your decision process. Don't rush anything. Don't auto-pilot.
All this sounds super easy but you'll see it is stupidly difficult to implement.