How are you traveling? Are you taking frequent trips or are you on a long-term journey?
For me, I'm lucky enough to be in a position where I can take trips frequently (casual hours), but I can't do trips longer than a month because I have to go back to work. I did get burned out on flying to different time zones all the time, and the stress of the whole airport process in general. I decided to stay home for the summer months as a break, and picked up more work hours. Well now, I can't wait to be on the move again!
As for fatigue during travel, I get tired out from constant decision-making. I'm also an ambivert, so while I enjoy meeting and talking to people, I find that I get mentally tired out after 3-4 days. For those reasons, I plan on rest days by giving myself extra time than I'd need in a location, usually in a hotel room. I also try to keep day to day stuff as flexible as I can, so I don't get stressed and can be spontaneous. I've taught myself to ignore the FOMO feeling, and to not force myself to do things when I feel that slump. It does go away after 1-2 days for me, and I get the urge to explore and socialize again.