"ended up turning a quick weekend project into a two-week ordeal."
Haha, I feel your pain. When I tackled my first window replacement last summer, I thought it'd be straightforward—just pop out the old one, slide in the new, and done. Nope. Found some pretty gnarly water damage behind the frame that had been quietly festering for who knows how long. Ended up having to replace part of the sill and even some studs. Definitely wasn't prepared for that detour.
Honestly, from what I've seen and heard, hidden rot or termite damage seems pretty common in older homes. It's like peeling back layers of history...you never quite know what's lurking beneath. Don't beat yourself up over it; you're definitely not alone in this struggle. Just think of it as earning your homeowner stripes—at least that's what I tell myself when things go sideways!