I actually did the thing with Anki, three languages at the same time, and it failed just like you described.

I suspect that it was an instance of a more general harmful pattern in my life, how I unconsciously turn my successes into future punishments. A pattern that I learned at childhood, and it's difficult to overcome, because at the moment it feels like a virtue.

Here is the pattern: I notice myself doing something right, and instead of just enjoying the situation and rewarding myself mentally, I feel the impulse to increase the burden until I break, which then provides me an opportunity to punish myself mentally (feel disappointed with myself). Which means that in long term, I am punishing myself for doing things right.

At the moment it feels like the right thing to do: I have finally found something that "works" for me; why not use it more? Just think about all the opportunities!

But the problem is that things don't scale linearly. I have a limited amount of time / energy / attention / whatever, and maybe the method already consumes as much of some resource as is sustainable. Another problem is that there is a difference between approaching a "#1 problem" and "yet another thing that should be done"; the former motivates to creatively expore solutions, the latter just creates an ugh-field around anything you use to push yourself.

Sometimes it is necessary to accept that there are many things I would want to do, but maybe at the moment I only have enough resources to do one of them properly. And I should look at the bright side and be happy that it is one thing instead of zero.

With the examples from article, in (1) I would recommend staying with French, and perhaps when the Anki workload is smaller, do something else in the remaining time, such as read a book or watch a movie in French. Switch from learning to using, without increasing time. Until it becomes a part of life (e.g. Bob would start regularly reading some French web pages) so it doesn't require conscious maintenance. If he isn't equally passionate about German, maybe he shouldn't learn it at all; maybe he just doesn't have enough time and energy for all that. In (2), Sally could use a weekend or take a day of vacation to look for the new job.