What should i do about my female friend thats let me down?

she sounds like the type of person that just takes takes takes in a friendship and does'nt give anything back. She's basically got what she wanted from you which was and now that side is basically done with, she does'nt need you any more. I'm sorry she's stung you like this, but you may need to just forget about this girl and try finding some new friends. Just dont make the mistake of sleeping with them as that really causes problems in a male/female friendship which you've unfortunately experienced. And you're right, if she was like any good friend, she would've remembered your birthday.