You say you want to at least become friends....but let's be honest, you want more than that. And with this in mind, I will remind you that she has a boyfriend and that you need to respect this and leave her be.
She's only 15 or 16. And being so young, the relationship has very little hope of lasting the distance. Wait in the wings till she's single again and then make your move.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.