Feeling terrible how? I sometimes feel bad waking up from a mid day sleep, or if I sleep late in the morning. I've also felt really, really bad if I take a multivitamin before sleeping, so I only take one if I'll be awake for several hours after.

Some would envy your lack of appetite. Food should be treated like medicine, so say some of the great saints. You can increase your appetite with regular carb/sweets consumption, and snacks throughout the day, though I generally recommend against doing such. Hard work can help too.

As long as you aren't loosing weight, or too far under weight, I wouldn't worry about it, and rather thank God for the limited appetite.

How do I decrease mine! 😂

I have been struggling with my appetite for a little bit now. I am starting to suspect it is partly due to me not getting enough nutrients. On Sundays I barely eat and even when I feel hungry I can't get myself to really eat much more so I often resort to just having glasses of milk or protein drinks. Does anyone have any suggestions for increasing my appetite via diet? I feel it is time to make a change as I often wake up feeling terrible too.

Adding physical exercise activities to your daily routine should do the trick.

What are you eating for breakfast lunch and dinner now?  

I think fasting for long periods would decrease appetite. It does for me.

Shouldn't lack of nutrients cause you to become more hungry?

Try eating some liver or other organ meat. If you consume a lot of liquid or very soft foods, like milk, ground meat, juice, or soup, your stomach will usually stay contracted from not being stretched out so much from more substantial foods. That could be it.

Maybe you simply don't need that much food, if you are very sedentary.

Could also be endocrine/hormone dysfunction. Check your blood sugar one of these days 12 hours after a meal.