Man, that peony story gave me flashbacks to the time a client wanted lilies swapped for orchids with 48 hours notice (I still have stress dreams about that delivery van). What helps me is having a firm policy sheet I email with every quote - it says last minute swaps may not be possible and lists a rush fee. For the anxiety part, I literally do breathing exercises behind the cooler when I feel panicked. In for four, hold for four, out for four. Sounds silly but it stops the spiral.