[Rivière]

To be honest, I don't really think there is any one real good answer a person could give you. The situation you're in regardless of the choice you make will hurt you either way. I think it boils down to what do you want more...

For them to know the truth and see how it goes.
To keep things inside just so you can be best friends with them.
To distance yourself and have them wonder what went wrong.

There is no good answer here, only what you want more.

What makes you say they will never love you back for many reasons? Have you considered asking them how they feel about their stance on relationships and falling in love?

You could make up a hypothetical situation in which you have a friend (you) who has fallen deeply in love with their best friend (your friend) and you don't know how to help them. You could ask their advice and support. Maybe consider asking if it were to happen to them (your friend) what would they do to help resolve the situation. This may help clear things up for you.