Help! How do I maintain a positive relationship with my dream collaborator?

So one of my dream collaborators responded to a cold call I sent out, and auditioned for my project.

I wholeheartedly believed this person would be a perfect fit for the project I am developing after months of looking at their past projects with a critical lens, but after the audition, and trying a lot of different ideas, I realised they were nowhere near as perfect a fit as I thought they were. Not that they were bad at their craft or anything, but just a very different approach to the project/their work even after notes.

How on earth do I let them know they didn’t get the part without ruining any chance of working with them in the future. Or do I just bite the bullet and work with them on it anyway despite my heavy doubts? I am thinking I’m a little bit clouded by my idolisation of this person (who frankly has been nothing but enthusiastic and lovely during the process), but this feels like a one in a million opportunity and I don’t know how to not ruin it?