Heck, I've been building disappointing rockets since the Nixon administration. Sometimes it was because an original design turned out to be more appealing on paper than in reality, or flew like a scalded squirrel. I've fubar'd finishes. I've fallen in love with a specific kit just to find out that the build was just not that much fun (the original Mars Snooper comes to mind). But...
I've learned new techniques. I developed an innate ability to understand materials and structural stability. I continue to improve my craftsmanship. I've built and flown things that most "normal" people have never conceived in their wildest dreams.
Yeah, I still disappoint myself. My most recent MPR, Blue Fire, is quite underwhelming.
Nevertheless, I keep on building and I keep enjoying myself.