How to tell your mum

I had sex at 13 and my mum would kill me if she found out, I'm almost 16 and have a boyfriend that's almost 18 I have been with him for about 4 months and I need to go on BC but don't know how to ask/ tell my mum. She always goes on about telling her when things start to happen but goes off about my friends that have already had sex, she is also catholic so she doesn't believe in sex before marriage.. What should I do?