Welcome here to the Forum. I hope you find it helpful. If you look around you may find others who have had to deal with similar circumstances and what may have helped them.
I guess I ought to point out that tiredness can come about for many reasons including diet, physical fitness, illness and medication as well as from a mental health condition or even overwork.
So first off may I ask if you have been to a doctor and been tested to see if the cause can be established?
To try to answer your second question first, how do you know if you are pushing your body too hard. I guess that is a judgment call based on the activities you are trying to do (do you have new harder tasks at the moment?) and also how you have been able to get along in the past.
With mental health issues again medical advice would be good. I do remember when depression was realy bad (I'm not suggesting you have depression, I'm no doctor) I found not only was I terribly tired but also had no motivation. Just getting up in the morning seemd impossible. Eventually I did it in stages, and saw each as a triumph, where before when simply not getting up at all I felt I'd failed -even though it was not me, but the illness.
As my mental health improved the tiredness lessened and my motivation increased.
If you would like to come back and say some more about your circumstances and what you are having to face that would be great