If it is as you said,...that you are "head over heels in love" with this girl,....then this is an anxiety/wish fulfillment type dream.
That she is wearing your current boyfriends clothing, suggests that you, or your subconscious, sees an attribute in your boyfriend that reminds you of that girl. Alternately, it could be that you secretly wish that the girl you love was in your boyfriends' place.
Her flinching is your way of expressing guilt for the way you're feeling...plus the fact that you're not entirely trusting of her, fearing she may hurt you again.

You'll have to do some soul-searching to find a decision that you can live with...to decide who is better for you, and your well-being.

Hope this gives you some insight, best of luck!