I constantly feel super tired

I genuinely cannot function on a day to day basis after any exercise that requires medium level efforts either physically or mentally, I can barely stay awake. It gets so bad I dont even want to leave the house anymore.

I have done every test over past year with multiple doctors but the only deficiency I have is a slight Vitamin D3, and I dont believe this can be the sole factor. My blood is fine, I get 9 hours of sleep at night and dont have problems falling asleep, I have a very healthy and balanced diet, I dont take drugs and I just dont know what to do anymore.

I have fought with depression in the past so could this be also caused by depression ane if yes is there any way to prevent it?