Where's Esmoreit when you need him?
I have no experience with dating other than crappy sitcoms having this same situation.
Here's what you do, both of you sing at the high school talent show (you and her bf), and watch to see who she chooses. If it isn't you, proceed to overdose on sleeping pills and plant your head face down in the toilet.
Kimi wa ne tashika ni ano toki watashi no soba ni ita
Itsudatte itsudatte itsudatte
Sugu yoko de waratteita
Nakushitemo torimodosu kimi wo
I will never leave you