My two cents: tam multa ergo feci, ubi non hoc erat velle quod posse [erat]
Taking velle as the subject, here is a literal translation to show how I understand the whole thing:
“I did so many things then, where willing was not this (hoc), which being-able was”.
Slightly less clunky maybe: “I did so many things then, where willing was not what being-able was”.
I admit I’d find the sentence easier to read (but less striking maybe) if id(em) had been used instead of hoc.