Quitting a collaboration project without hurting collaborators
Just graciously explain yourself and be firm. They may not be happy but if you just won't go forward, they have to deal with it.  

You can't always make everyone told happy.  Sometimes you just need to cut things off.  

Don't belabor or debate.  Just be respectful, but firm.  And brief.

https://www.youtube.com/watch?v=ABXtWqmArUU