"I have feelings for you and I can tell that you have them in return, but you are TAKEN. If, in the NEAR FUTURE, you are no longer dating this person, and I am STILL available, perhaps we could date and give it a try. However, I will not cheat behind someone's back as I would never want that done to me!"
You MUST make this person make a tough decision about whom they truly want to date and be with and let them know that you are not going to wait around for them to make up their mind.
Good luck. If you choose not to follow the advice given above, you will find yourself lost in the "Bermuda Triangle of Love" and in a love triangle, someone is always the "odd person out." In this example, that will almost always be you!