The cohort, replaced

The last missing piece is other people, and it is the one most often skipped. A cohort supplied a pace, a comparison, and somebody to ask when stuck — and being stuck alone with nobody to ask is where most self-directed learning quietly stops.

Replacing it does not take much: one person doing something similar, a standing message once a week, a group where questions are normal. One other human who knows what you were meant to have done is worth more than any application, and it is the cheapest of the four to rebuild.