Hi chula - for those of us who are a bit older, it used to be exactly like what you describe, for everybody! It can seem a bit strange when I see so many teenagers are having sex at 14 or 15, with their parents permission, even having their boyfriends stay over!
But you are 18, and you sound like you are pretty mature about what you are doing, and that you have a pretty good relationship with your mother even if you don't think she'd be okay about you having sex. The fact is, she is probably perfectly aware that these days you aren't going to wait until you are married, and she may sort of know you already are doing it. But there's no point in rubbing her nose in it, right?
Anyway, I was on the pill living at home, when I was 17, and it is possible to be discrete about it. It helps if you know your mother doesn't snoop through your things! The trouble when you have to hide it, is remmembering to take the darn thing every day, if you can't keep them someplace where you'll remmember like next to your toothbrush. What you'll just need to do is keep them someplace she won't see them but keep a reminder someplace to make sure you don't forget, because that is the main thing, is to take them every single day without fail!
Good luck!