One thing about fluency that seems obvious but isn’t, is that your proficiency in a language does also depend on your mood, confidence, and energy level. There are times when my brain just can’t handle the additional workload of translating things or operating in a different language because I’m sick, or out of it, or even just hungry. Sometimes it depends on stress and fatigue.
Ideally you’re in a position to take breaks, but it’s really easy to get bogged down by mental exhaustion. It’s also really frustrating because physically, I can get super motivated and ready to go, but mentally drained.
I guess in other words, sometimes you just have bad days. Or bad hours. You kind of have to get good at understanding what your body wants and needs to do things long-term, or you might end up coasting between bursts of inspiration