First of all, congratulations on finishing your UFOs (all but one)!! That is a great achievement.
It sounds like you have made an honest effort on this one and then some. Yes, it is okay to stop working on this quilt and to use the remaining fabrics for something else. One of my Dad's rules for poker that he taught me was, 'don't throw good money after bad.' The same could be said for time and energy. Your time and energy are precious, you don't have to keep giving them to a project you feel was a mistake from the get go. Let it go and move on to something you find pleasure in. And think of all the projects you did complete!