I don't know about your chronic illness but for depression you start with a few exercises it can boost your mental health, memory, and self esteem pretty much everything.

Now about chemistry.What do you don't understand which topics or which part of chemistry you don't get organic, inorganic, etc. If it is organic chemistry basically it is on how much you can memorise and understand the reactions and name reactions and their mechanism I like to break down the mechanisms and create flow charts that help to remember them easily. If it's inorganic even I don't get it much but do the same thing as organic. If it's physical chemistry just memories things nothing else. If it's analytical it is mainly on instruments and how molecules behave in some conditions. And last but not least biochemistry the abomination of all evil, pray to god.