I need to convince my mum to let me on the pill

Okay so long story short I’m 17 years old, when I was about 13 or so my mum gave me the talk yous know how that goes... anyway so at the end of that talk she goes “as soon as you start having sex tell me so we can get you on the pill”... so this is my problem- she knows I’ve had sex and she knows that I want to go on the pill however she thinks I want it so I can have sex with anyone at anytime which it’s not the case at all! Currently my period is 2 weeks late (and I know I’m not pregnant cause I haven’t done anything for yonks ) anyway and I recently had 2 outdoor camps which were practically deserted and I was supposed to have it then too but thank god it was late then also.

The point I want to make to mum is I wanna be able to know exactly when it’s coming and going and if I am in that situation where I was going on a camp or something I could have it early or later. (And I guess the saftey would just be a bonus too.)

Anyway I need to convince mum can you girlies please help me out and give me some good points I can throw at her xxxx