I agree with what you said, but I'm also thinking, re-reading what Xiphius said,

[...]as I went through the trip and went on doing things because I'd not have another chance to do them, and besides, I didn't want to hold other people back, and besides-besides, I really actually would enjoy those things, I knew that I'd be spending at least a day and a bit bedridden for every day I pushed through on willpower on the trip.

I wonder if next time you're on a trip like this, where there is a villa/home base, for you to decide beforehand that it's OK to say, "Hey, ya know? Y'all go out to [tuscan village/ cool day trip] today; I'll stay by the pool, surf the net, or [insert fun, but non-stressful thing here], and rest up so that I I can [do fun things X and Y / walk without pain / avoid collapsing] tomorrow.

If you worked this out with Lis and your family beforehand, maybe it wouldn't have as large an impact on you afterwards. And now that you've been to Italy once, the next time you won't have to do everything that you did this time, so that would be less stress.