I do lots of planning as well, spread sheets, photocopies of reservations, train tickets, etc. I am most organized when traveling! I find that the research and planning is part of the journey and does relieve some stress.
I also avoid changing planes at JFK whenever possible. And I spend the $$ for economy comfort seating; in flight I feel like a sardine rather than an anchovy.
I get stressed over things like having enough startup money, so I try to make sure I have enough for the first day - or at least to get me to my hotel (usually in city centers so there are ATMS everywhere).
Then it's just breathe, pinch myself that I'm in Europe again, and remember that some of my best travel stories are when things went wrong. The stress is there in the moment, but it disappears, as noted, with a capuccino (before noon) or something stronger later on.
It helps me to remember that traveling is a privilege and that what may be an inconvenience to me is what others live with every day.