I'm 15 and I want to start using birth control.

I have a serious (yeah I know I'm 15 and relationships that started in middle school usually don't last. I. Know.) relationship with a guy for about 2+ years now off and on. 
I want birth control but I don't want it only for the reason of not having a kid. I have had sex with my boyfriend and while we try to use a condom..it doesn't always happen. 
I haven't slept with him in like 2 months because he never has a condom anymore. 
He uses the excuse of "well if it hasn't happened by now.." And I always respond with "yeah you say that till it happens!"
But this isn't about my teen sexlife. Not fully anyways. I want birth control because well, there are other uses. And I just want that extra protection and peice of mind. 
Nows my problem. 
I life alone with my dad. He doesn't know I have sex, and he is the kinda parent that thinks birth control is only for kid prevention. So he would never allow it because that means he allows me having sex. 
I don't know what to dooo.