slap his bloomin face :P
find another girl!
Dump the girl. If she is in love with another guy then she doesn't deserve you. Don't waste your time on some one who does not love you.
You should reconsider if you really "love" this guy.
no
carry on find another girl
kill her kill her
Forget her and move on.
Is he your boyfriend? If he is, then talk to him about it. If he isn't the you should confess to him that you love him.
If a girl loves you and loves another girl then your basically dating a bisexual
There are no specific reasons why as the person loving another have their own personal reasons why they love someone.
If they are in love with another girl then you should leave it alone completely. Eventually you'll find someone who is not involved with someone else and you don't hurt anyone, including yourself.
You should not be in love with a stubborn girl.