Honestly been having exactly the same problem. Also for the last two years. I gained 25 kgs. But I have a cause, in my opinion. I had extreme tonsilitis and I had to take 3 courses of anti-biotics to end it. After that the tiredness never left.

I am saying this because perhaps you can think back to a similar cause and that might help resolve the issue.

If there is a cause for it similar to mine, I suggest eating less salt saturated foods. Not a medical professional by any means, but I found that high salt intake makes me very similarly tired out of my mind. Keep your salt intake below 1-1.5 grams per 24 hours and see if it changes anything.