Severe occasional fatigue. I am 80 but usually am full of energy mental acivity. Am tryig testosterone but not helpng?

Maybe not testost. Don't try testosterone unless it was really low as determined by a test. Fatigue is usually not as simple as replacing testosterone. Could be you just need more rest.
Low T. Please be certain you need the testosterone. Also fatigue should be evaluated with cardiologist and blood tests.
Check T level. Lots of reasons to be tired/fatigued including anemia, hypothyroidism, heart/lung/kidney/liver disease that all need to be evaluated by your family doc or geriatrician. Taking testosterone only helps 1) if you need it 2) if you take enough 3) if it's not wasted/converted into estradiol. Too much t is bad (side effects) but not enough isn't good either. You want just the right amount for you.
Discuss with your Dr. I applaud you for doing your best to keep fit at 80. Hope you are getting enough sleep. You may well need to have a rest or even a nap during the day. Consume a well balanced diet without too many calories, hope you are not overweight, don't smoke and don't consume excessive amounts of alcohol. Good luck.
Occasional fatigue. You are eighty and get occasional fatigu, and I suppose your testosterone level was low as is expected at 80.Did you check your total and free testosterone level, and I presume you are on the right dose as prescribed. Occasional fatigue at 80, can be from many other causes, if testosterone suuplement is able to bring your level up and you still have fatigue, you may have vitamin deficiency and do levels.