How do I keep track of why I made each decision?

You have learned to write your decisions down. What still trips you is capturing the why, especially when the decisions come fast and thick and there is no time to explain each one. That is a real bind: the reasoning is clearest in the moment and gone by the time you have breath to record it. Under volume, the rationale is the first thing to slip. The thing to accept is that you cannot capture everything, and trying to will just make you stop capturing anything. The reasoning behind a small, obvious decision is not worth the same effort as the reasoning behind a big, contested one. The skill is not writing more. It is knowing which decisions will be questioned later and spending your few words there. So build one small habit: when a decision feels like one someone might challenge down the line, stop and write a single sentence, why this and not the obvious alternative. Just one line, just for the ones that matter. You are not trying to document your whole mind. You are leaving a trail on the handful of choices where the trail will actually be needed.