What am i supposed to do ?

You need to calmly explain to her one last time your situation, and if she is truly your friend, she would understand. If she still insists upon you lying, then you two just need seperation. No contact for a bit, and I promise she WILL come around, or at least be the first to talk to you again. You are in a hard situation, and it's tough, not wanting to lose a friend, but if she were a real friend, she would understand, or at least come around later on.