Besides what @Anamana@feddit.de said, it could be a vitanim deficiency, there are periods of time where I feel like that and taking vitamins help me, the one I am taking has Vitanim A, C, D, B12, B6 Gingko Biloba and Ginseng, among a bunch of stuff like iron.
Before playing around with that, I would suggest OP to get their blood checked via hemogram. This way you can usually let doctors determine deficiencies and subsequently adapt your diet/supplements.
There are many reasons so no one solution fits all. If you don't know how to fix it yourself a competent therapist can often help to find the root cause and ways to address them.
For me it's working on my own projects instead of working for somebody else. The difficult part is making money from it. Working 40h on what I am passionate about is far less tiring than working 30h in some office.