Have you ever devoted a significant amount of time to something only to later feel it was pointless?

Not quite. I do believe values and existence in general are meaningful, but I'm convinced that mine specifically don't matter. Nothing I've ever done had any impact on anything or anyone, so I am either just a gigantic loser (likely) or all others are idiots (unlikely). I would be exactly at the same shitty spot in my life if I didn't try to uphold my values, so why bother anymore?