When I am sightseeing, I'm always tired by mid-afternoon and I'm not sure why. At home, I'm on the go all day at work and I'm not at all tired after work. Going to an evening event is no great chore, and I never feel the need to lay down and rest after work.
But send me on vacation and I'm tired all the time! I don't know if it's physical or psychological. Indeed, it may the the mental stress of finding my way in unfamiliar surroundings that tires me faster when I travel.
Or perhaps it's the walking. At home, I walk around the office some but otherwise it's just the walk from my house to the car (30 feet) and the walk from the car into the office (30 feet). When I travel, I'm walking everywhere. So maybe taking bus tours rather than walking tours would help.
Anyway, what works best for me is to stop the sightseeing about 2 p.m. and head back to the hotel for a shower and siesta. Then I'm re-energized to go out for the evening. The shower is a MAJOR help in getting rid of the feeling of fatigue.
If I were in your situation, I'd stick with my plans as far as what cities to visit, then take it one day at a time. Cut back what you'll do that day if you get tired. You should be able to see what you most want to see, but cut out lesser sights if you don't feel up to it.