You shouldn't blame a particular programming style for the failure. Your real failure was an organizational one.
It seems that you have more experience with the imperative style than the functional style. And yes, it's always good to try a new style you aren't yet comfortable with, but you shouldn't do that in an important project. Also, you might have had better luck with Haskell or Lisp rather than C++. But here again, you shouldn't do that in a time contrained project if you aren't fit in that language yet.
Instead, you could have taken the week to implement it in the style you're comfortable with, and rewriting or refactoring it into another style to play around with it. That way, you would not only have eliminated the risk of failing. You would also have learned more, being able to compare both styles side-by-side.
It seems that you have more experience with the imperative style than the functional style. And yes, it's always good to try a new style you aren't yet comfortable with, but you shouldn't do that in an important project. Also, you might have had better luck with Haskell or Lisp rather than C++. But here again, you shouldn't do that in a time contrained project if you aren't fit in that language yet.
Instead, you could have taken the week to implement it in the style you're comfortable with, and rewriting or refactoring it into another style to play around with it. That way, you would not only have eliminated the risk of failing. You would also have learned more, being able to compare both styles side-by-side.