Re: Question
Your situation is understandable. You messed up when you became his best friend. If you analyze it properly, you are not being sincere with your fiance. Nothing beats the feeling of having a fling going with your crush and at the same time having a certificate of secure future - which is the engagement in your case.
A friendly advice - stop being dishonest with yourself, your fiance, your best friend and your crush. Get yourself out of this situation and focus on your studies/career and your future.