What actually breaks a training streak, and how do people recover from it?

It is rarely a dramatic event. The usual sequence is a genuinely busy week, then a second week where the habit has lost its slot, then the feeling that restarting requires going back to the beginning. Two things help. First, define a minimum version — ten minutes, one exercise — that counts on bad weeks, so the chain never fully breaks. Second, accept that a two week gap costs almost nothing physiologically; strength drops very little and returns quickly. The cost is psychological, and it compounds only if you treat the gap as a verdict.