We don't usually experience hallucinations or delusions without some external chemicals. So as long as your not you should be in control of your brain and the thoughts that are contained in it.
A common though, unfortunately unanswerable question. Neurochemistry has a huge influence over our thoughts and actions, however it is not clear to what extent. Whether or not we actually have freedom to think and do what we choose is unknown. Only further research can determine if we are truly free or not.
I used to long distance bicycle race, the annual event was a 700 mile race. I should reiterate, it is a bicycle, you pedal it to make it go. You need to be close to 6 hours per hundred miles to be in the running. 7 hour centuries put you at just over 48 hours. Hallucinations are common during this race. Most riders see a wall or a fence across the road and they just stand there as their path is blocked. I however am an eternal optimist. When did hallucinate, which happened once during one of the rides, I hallucinated a bunny rabbit running beside me for like 60 miles. After the first hundred yards I figured out it was not a bunny rabbit. I still enjoyed his company even though I knew he was just a hallucination.
So the optimist hallucinates positive things?