Hi, I'd like to help you with your question.
It sounds like you have already done some research to help yourself get a better idea how to combat mental exhaustion. I hope I can add something new to help you.
Here are some steps you can take to help yourself:
Get enough rest. Make sure you get between 7 to 9 hours at night or most nights.
Eat well balance meals and if you are working hard, make sure you snack on high protein low sugar foods to help keep your energy up.
Make sure you are exercising. Exercise not only helps keep your body healthy and fit, it gives you a chance to do something that is physical and not mental.
Make sure you take time to relax each day. Even if it is just one half hour, do something that does not involve mental energy. Watch TV, do Pilates/yoga, etc.
Laugh. Spend time everyday trying to find ways to laugh. Watch a comedy show, joke with friends, play with your kids/dog. Laughter promotes good endorphins and helps you relax.
Take an Omega 3 and B vitamin each day. Check with your doctor first. Both supplements help promote stress reduction and B vitamins help reduce stress reaction in the body.
If you find that nothing you try is helping, consider talking with a counselor to help provide you with support and so you can learn better ways to help yourself cope. Talk with your doctor for a referral or if you attend church, your pastor can help. You can also search on line at http://therapists.psychologytoday.com/rms/.
Here are some books that may help you as well. One is called The Exhaustion Cure: Up Your Energy from Low to Go in 21 Days by Laura Stack. Another one is The Relaxation & Stress Reduction Workbook (New Harbinger Self-Help Workbook) by XXXXX XXXXX, XXXXX Robbins Eshelman, Matthew McKay and Patrick Fanning. You can find these on Amazon.com or your local library may have them for you.
I hope this has helped you,