Can success be depressing?

After you've finished/accomplished/beaten something you're really proud of, have you then realized that it's a pretty ordinary thing you just conquered? I don't know whether I should (a) feel warm 'n' fuzzy about what I've done or (b) feel like a moron for thinking it was hard enough that I should celebrate its completion.

Is it okay/justified/good to be proud of something that's not unique or rare?

Is success relative or absolute? Is a relative success more motivational or less so than an absolute success?

Vague, yes, but it's a big theme in both my personal and my professional lives right now.