Strength bar losing strength.
Even practicing everyday my lessons, and with almost with no mistakes, my lessons keep losing their Strength, so instead of moving forward, I have to always come back and get their Strength up again, I thought you would lose strength by not practicing, but with daily pratice, really? This is really stressful, is this normal?
Strength is lost over time too. Periodic review is necessary and Duo enforces it in its own way.
If you are confident that you have mastered the material, just keep moving on to the next skill/lesson (which will have been unlocked anyway once you finish the prior skills). You can always review material as needed.
FWIW, I first finished the tree, and then went back and made all the skills golden. It's been vastly easier now to maintain an entirely golden tree on a daily basis.