I sleep a lot but still wake up exhausted, what could be wrong with me?

Sleep. If not already recommended, see a sleep specialist to help you. Obvious considerations are sleep apnea dn narcolepsy but other possibilities include poor sleep habits/hygiene, environmental noises, psychiatric disorders, rls/plms, fibromyalgia/pain syndromes, etc.
Lots of ideas. Daytime sleepiness can be causes from many issues including sleep apnea, restless leg syndrome, poor sleep habits, medication side effects and depression to name a few. You should consider seeing a sleep specialist.