Not being able to do Lamarck (4x10 at FTP) should have been your sign. Any time you can’t complete steady state sweetspot or FTP work usually means you either aren’t eating enough, or you need rest.
Usually I rely on mood. If I am tired and not motivated to even do an endurance ride, it’s time for an easier week.
If you want a deeper dive, post a public profile link or Strava and a few of us can take a look.