[GianFabien]

I went through a period like yours. Seems that there are various (unknown to me) factors that influence the severity of the condition.

What worked for me was:

1. make a decision to stick with one task at a time. But also have a pad at my side.

2. whenever a distracting thought strikes, jot down the barest reminder about it on that pad and go back to working on the task.

I don't know whether it is related, but I also took up journalling, aka morning pages, around the same time.

The key thing being both the pad for jotting things down and the journalling are done with pen and paper. As much as possible I get away from the computer and work/think/write where there are no screens.