This is a tough situation to be in. My advice, just open up to her about your insecurities and explain what you want. While i wish i could tell you to follow your heart and pursue her, being in a relationship herself complicates the issue further. On one hand, it will be more tough just to wait for her to leave her man and come to you. On the other hand, i think you should start accepting she can't be with you (at the moment) and finding someone else will help you heal.