Duolingo assumes I forget way too fast
I have been using the german lessons for more than a year now and it appears to be impossible to fill the whole tree unless you have a really high daily goal, like 100 xp perhaps. I have set mine on 20 xp and that is not enough, i'm on 49%. Every day there is something to strengthen, and if you miss a few days you have a huge backlog. The other day it even got back to the basic "eine Frau" lesson. It is so annoying to have to strengthen one at a time. Unless there is a way to test out on groups of lessons you have already done that I have missed, I think I will uninstall this app now. How do you really reach 100%??
I think the best solution at this stage is to start ignoring Duolingo's defective strength algorithm and accept that a golden tree isn't worth the effort any more. If you've completed the tree, it's a good time to start the reverse (English from German) tree to get more practice writing in German.
A "Duolingo Fluency" of 49% in level 16 is a very good result!
Please read point 2. in this explanation
You don't have to have your tree gold! I like the approach in this discussion
On the contrary, I often find myself repeating lessons that are still golden but I am not completely comfortable with, and I have never been forced to repeat the simple lessons. I find speed review useful for a quick refresher, it often takes just 2-3 min to get 20 xp. When you do the general "strengthen skills", you might also strengthen more than one skill. Take your time. Your tree won't be done soon. It was helpful for me to plan on fluency in about ten years, thus not worrying about how much progress I make in a single day, just that I make some. I'm not in a rush. Rush creates stress.
You can't reach 100% - the fluency is a flawed metric anyway, but even Duolingo don't claim that they can get you to complete fluency. Overall, I'd suggest ignoring the fluency meter because it's fairly meaningless, but trying to get it to 100% is a fool's errand.
If you make very few/no errors and don't use the hints, your skills will decay much more slowly.