I want to continue to be someone who thinks things and does stuff, even at a cost in happiness.

The FAI can make you feel as though you "think things and do stuff", just by changing your preferences. I don't think any reason beginning with "I want" is going to work, because your preferences aren't fixed or immutable in this hypothetical.

Anyway, can you explain why you are attached to your preferences? That "it's better to value this than value that" is incoherent, and the FAI will see that. The FAI will have no objective, logical reason to distinguish between values you currently have and are attached to and values that you could have and be attached to, and might as well modify you than modify the universe. (Because the universe has exactly the same value either way.)