Right after starting a post-doc, another post-doc in the new group came and invited me to collaborate in one of his projects (only a side thing; not my main research project; somehow independent from my advisor). I agreed to participate, but recently I've been procrastinating that work because, to be honest, it doesn't seem quite appealing now.
Of course, I have other projects of my own -which are going ok-, but this one is a bit of a bug in my head. How can I gracefully tell my colleague that I'm quitting?