I don't really know why you feel the way you do, but honestly it's vital you accept yourself as you are, and validate your own self.
You love both men.
They love you.
What about either of those things makes you a "slut"? A "whore"? I know we all have deprogramming to do when it comes to living poly, but you come first, and how you see yourself trumps everything. How long have you been feeling like this?