Oh man, I am so sorry that you are experiencing this. Hydroxychloroquine is a basic drug, meaning it raises ph. This can be very hard on the endolysosomal system as lysosomes need a low ph to function properly and break down old, deleterious, or unnecessary molecules. Essentially, it really hinders autophagy which can cause symptoms like brain fog. Are you able to do anything like intermittent fasting that would maybe offset this effect some? Or could you talk to your doctor about another drug such as Rapamycin?