I have a crush on my best friend

What did she say when you asked her out? If she said no each time, then I guess she only wants to be friends with you for now, so stop bugging her. If she said yes, then great. Take her out. If she said maybe or was unsure, then give her time. If she doesn't accept your date invitation in a reasonable time, then maybe it was her way of letting you down easy. So go find someone else to crush on.

I would just add (and the advice is excellent as always. G'morning, CoastalRat) that you may be somewhat idealizing her because of her choices in relationships. Plus you might be behaving unfairly toward these men (I am assuming you are female, so this is a same-sex attraction; please correct me if I'm in error).

The fantasy of 'saving' someone can be a rather powerful one, even if they don't need or want saving, and even if the rescue that's being offered is sexual only.