I think the nuance is as follows: 1. I am not usually happy --> This means that you are not usually in a state of happiness. This doesn't exactly mean you're sad or upset, only that you feel something else or nothing at all most of the time (stress, excitement, etc.). The "not" applies to "usually". 2. I am usually not happy --> I take it as: I am usually unhappy (= not happy = sad). The "not" applies to "happy". I hope that was helpful.

Hi Rali how are you? Both sentences are grammatically correct! Either one you can use!🍎