As a guess, then, you might need a somewhat "racier" riding position for climbing - your current position is maybe demanding too much of your lower back muscles at the higher average torque of hill climbing - but that might compromise your comfort in the flats. Adjustability is probably limited there, though, unless you have an adjustable stem. To the extent you can - maybe flip the stem, adjust spacers, etc. - try a slightly lower bar position to see how it affects your climbing comfort level. Another consideration might be your reduced energy level after a day's work, depending on the nature of what you do. I leave work exhausted most days, and it definitely informs how I choose to ride home.