I take L-Citrulline and Arginine with Vitamin C to increase nitric oxide production plus Creatine to increase energy. After several months, I have been back to normal. Here are some references to them:

Fatigue. Nitric oxide decreased levels may cause chronic fatigue. Low nitric oxide levels can decrease your body’s natural energy at the cellular level. This is a result of reduced circulation, reducing the delivery of oxygen and nutrients that your body needs.

Our bodies produce nitric oxide in the lining of our blood vessels and the saliva / bacteria in our mouths. As we get older (us “Seniors”), the blood vessel lining stops producing nitric oxide and we are dependent on our mouths producing it. A 40 year old is producing 20% less nitric oxide than they did when they were younger. At my age (80), it is 75% less. The 25% that is produced is from our mouths. (Remember, no mouthwash or fluoride toothpaste that reduces the good bacteria.)

Which means the right diet is very important. We need food with nitrate / nitrite that the bacteria in our mouths convert to nitric oxide. So beets, arugula, blueberries, pecans, peanuts, cocoa, kale, bok choy, celery and others are good sources. So think in terms of cocoa for breakfast, salad (arugula, beets, blueberries, pecans) for lunch, and a bok choy stir fry for supper. Maybe peanuts for a snack. It does take some meal planning and concentration on having what works, but it is worth it.

From the National Institute of Health:
Our survey indicates that the supplementation with L-Arginine + Vitamin C has beneficial effects in Long-COVID
Following a 30-day treatment with L-Arginine + Vitamin C, the survey revealed that patients in this treatment group had significantly lower scores (which means less severe long-COVID symptoms) compared to the other group (L-Arginine + Vitamin C: 8.15 ± 1.3 vs Alternative treatment: 13.9 ± 2.3, p < 0.001); remarkably, the treatment with L-Arginine + Vitamin C had favorable effects on all the symptoms explored by the survey
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9295384/
From Medical News Today:
The study found that taking dietary creatine for three months substantially improved feelings of fatigue, and by six months, had produced improvements in body aches, breathing issues, loss of taste, headaches, and problems concentrating — or “brain fog” — compared to people given a placebo.
https://www.medicalnewstoday.com/articles/can-creatine-supplements-help-people-with-long-covid
From Healthline:
Citrulline produces several important effects in the body. One major way it works is by increasing vasodilation.

Vasodilation refers to the widening of arteries or veins. It’s associated with lower blood pressure and increased blood flow (5Trusted Source).

After citrulline is consumed, some is converted to another amino acid called arginine.

Arginine is converted into a molecule called nitric oxide, which causes vasodilation of blood vessels by relaxing the smooth muscle cells that constrict them (6Trusted Source).

Interestingly, consuming citrulline may increase arginine in the body more than consuming arginine itself (4Trusted Source).

This is because of differences in how the body processes and absorbs arginine and citrulline (2Trusted Source).

The increase in nitric oxide and blood flow may be one of the processes involved in citrulline’s beneficial effects on exercise performance.
https://www.healthline.com/nutrition/citrulline-supplements#TOC_TITLE_HDR_3