Practically speaking, i would first look at the food you are putting into you. Make sure you are getting a balanced diet with sufficient nutrients and calories to maintain your lifestyle. You don't need to get obsessive about it, just as long as you are getting mostly carbs, some amount of protein, a spot of fat and prioritize fresh produce to deliver the vitamins.
Second i would look at your sleep health. Try to get 6 hours uninterrupted every night, ideally closer to 8. If you are waking up a lot due to environmental conditions, that isn't going to be enough to let your body recover from the day before. It might not hurt at the start but takes a toll over longer periods.
Third i would suggest to go to a doctor and get a standard physical just to see if there are any obvious imbalances that could be explained by disease, especially if you have been traveling in the tropics. I hope you already got all of the vaccinations on offer, steered clear of mosquitos, scabies and other critters, kept your body and belongings clean, and that you don't drink, smoke, do drugs and so on.
This is all basic personal health and hygiene stuff that every person in the world, traveler or not, should know how to do. If you are doing all those things, the doctors didn't find anything, and you are still tired, it might be psychological. That's a tougher thing to solve, and i think it's different for everyone.
There has been a few phases in my life where i got paranoid i did something that permanently scarred me because of unexplained exhaustion, ennui or pain. Sometimes i was in a position to visit a doctor and go through batteries of tests and was fortunate to find that nothing abnormal was detected. Not to diminish the subjective feeling of fatigue, but just to put into perspective. There are a lot of hardworking people who are severely malnourished or are suffering from disease, have disorders of the immune system, other chronic illnesses, permanent injuries and so forth... If the worst thing you have to worry about after living a life on the road is a sense of tiredness, you're coming out on top. Knowing that was a comfort to me.
Last, but not least, age. When you get old, your body starts falling apart. You can't do all the things you used to do when you were young. Everything is harder than it used to be. You need to go to great lengths just to maintain the same level of health you had before. It fucking sucks, but sooner you come to terms with it the better, because it's only going to get worse.
