Hot 2 convince gf to give it up?

Ight im 14. Im tryin 2 get my girl 2 give it up and she says she will but weve chilld a couple times and she changes her mind. Wtf like no other girl ive dated acted like ths. And she says shes nervous and doesnt wanna get hurt. So how do i fix this lil problem and get me some... PS: Save the moral right and wrong **** for someone who cares...