Sounds like depression to me.
I suffer from bouts of that too, generally when I've been battling issues, or rather, not dealing with issues for a period of time. They all seem to take their toll on me as I continued to push them deeper and deeper inside. I isolated a lot too! Still do to a certain degree. But, I also know that I require much longer periods of solitude than most people I know. That's just me!
Sometimes you really do have to force yourself to do something, anything constructive for yourself. You get to choose what that is. Even if it's just for 5 minutes or so.
One of the things that got me going was redecorating my apt. (on a budget) after my ex left. I made it more of my style. I painted furniture, rearranged the furniture, bought plants and nice pottery, pillows etc. Sometimes just going out to the store for the paint or for decorating ideas was helpful. It got me out into the fresh air, interacting with other people, and for those periods of time, I didn't feel tired. I was actually energized.
When I stopped doing those things, at times, the depression and fatigue would reappear. But I learned how to 'put it on the shelf' temporarily at least by keeping busy with something that interested me. Over a period of time, I worked up to tackling the issues that were bothering me. Again, a little at a time.
You need to find something that interests you. Start out small, for short periods of time, and go from there.
Be sure you get your rest, eat well, take vitamins, drink plenty of water too. These are things that are easy to do, and do make a difference.