Have you ever had that feeling?

when a person you really like looks at you and you just feel like you were punched in the gut. but you also know they feel the same way. there is just such a profound connection with that one look into the eyes that you just know your going to be with that person. well this girl and i have this connection. the problem is she has a boyfriend i already know that i need to back off until it's over but i just cant be static and sit there i need to do something. please give me some helpfull and usefull advice.