My girlfriend flirts yet she wouldn't let me be. What do I do?

Hey dude, realise the girl is using you. Best thing is to use her too. Play around on her. Don't hide it. Girls always want what other girls have, that way being seen with her makes you more attractive and other girls will want you more. When you "let them" steal you from her this will hurt her and make them happy. Ha ha.
Don't think she'll change, tears are a womans best weapon (possibly more than sex).
Truth is she won't like you treating her like this so why should you put up with it from her.
Using her is the only way to escape with your dignity and maybe some new girls  

Dump her and pray 4 her (SA²BC):Simple As A B C.

dont worry, she is trying to kill 2 birds with 1 stone

Just out of curiosity when did flirting and cheating mean the same thing

   All I can say to you is that the writing is on the wall.You dont need to ask us what to do,you already know what you have to do.
   There is point being in a relationship where you are unhappy,have you heard that saying "You can comb a dog,bath it but a dog still remains a dog"
   If she is triffling now she will always be,I just pray you realise this before its too late and thats falling in love and you know who will be the FOOL then.She will walk all over you like an OLD RUG.
  Kiss her ass to the CURB,ladies are looking for a good man these days and yours has you but she refuses to see it.And besides she is greedy,wants to have all the fun while you stay at home.
   Let her know this fact,guys to sleep with are very very plentiful but a guy that will be with you through thick and thin.Through the hard times and lead to marriage well these are a Dime a Dozen.  By the way Men are scarce these days oh,at a time dem scarce like KEROSINE

Dude tell her to fock off.

Or do what Balrog has suggested