This is largely present bias: our brains weigh immediate, certain costs (the discomfort of starting now) much more heavily than future, uncertain rewards (feeling better later), even when we logically know the trade is worthwhile. Add in "ego depletion" — willpower that's genuinely harder to access when we're tired or stressed — and the brain reaches for whatever rationalization lets it avoid the short-term discomfort, even if it contradicts what we'd tell a friend in the same situation.