I am a helpful colleague.

That is an understatement.

I did and finished something which I thought was my task, but was actually W's.

W told S that the task was sort of done, doesn't know why, and left it as it is.

And I realised that I was the one who contributed W time to have tea breaks, maybe watch a movie or 2 and still not have to work after-hours, 10 minutes ago.

Now I might have to skip pee/shit breaks, not ordering drinks during lunch and work 30 minutes later than my normal after-hours, just to complete the task assigned to me orginally.

I feel stupid.

And that, is an understatement too.