What does 早待不住了 mean in 要在往常，我一定早待不住了……?
要是 signifies a hypothetical construction. 往常 does not only mean ‘as usual’ (which only corresponds to 常, as in 通常, 正常, 常規); 往 here specifies the past, as in 往日, 往事, 往昔. Together they mean ‘normally in the past’.

早 is an adverb meaning ‘already’. Think of that as saying, something happened well in the past (early in the timeline).

他早(就)離開了。 He left already.


待 is a verb meaning ‘to stay’. 不住 is an add-on to it, literally meaning ‘not to a still’. Figuratively, 待不住 means you can’t contain yourself, or you’ve got itchy feet. Perhaps you're excited or anxious about something.

The whole sentence means ‘I’ in the past would’ve not been able to control myself in coming up with the many racing thoughts in my head (unsolved cases, scientific breakthroughs, lottery etc.). My mind is so irritable that these thoughts would have occurred very easily and very soon (hence the use of 早). But now my mind is completely at peace, so they are nowhere to be found.