In the first place, you knew that she's still in a relationship yet you still didn't reprimand yourself from feeling whatever you're feeling right now. You tried to take advantage of the situation the girl is in with her bf and never thought that at any point, she'd still choose him over you.
No matter what the status the relationship between them is, the fact that they're still in a relationship remains the same. If you could have thought about yourself, even just for a while, you wouldn't be in this situation right now because if you love yourself, you'll never allow yourself to be just another person's option when you can be somebody else's choice.