Mom and Birth Control

So me and my boyfriend have been getting really serious lately and have even have had sex a few times (yes we've used protection) but the problem is that I'm not on birth control. I'm 18 and my periods are really irregular so birth control would help in many different ways. But my mom has me on her health insurance and every time I ask her if I can be put on birth control she freaks out and basically holds the insurance over my head. I get it, I'm her baby and everything but it's starting to be a problem because with me getting serious with my boyfriend I don't want to end up pregnant because I'm not on birth control. And he is even nervous to have sex with me because I'm not on birth control. And even my sister was in birth control at my age. So my question for you ladies is how do I talk my mom into letting me get on birth control?