My Dad, with my Mum by his side, told me. Mum was worried that I would end up thinking I was dying like she had. Yet she was nervous and had no idea how to approach the subject with me. Dad is very open and funny so 'the talk' was both embarrassing, bewildering and very, very funny! I remember asking Mum after, why on earth she hadn't just told me herself. Saying that my Mum didn't keep well and was often in the hospital, so having that openness with my Dad was very handy when I needed pads. He was happy to buy some in the shop if I needed, which caused some hilarious phone calls! He always bought me galaxy chocolate, feminax and warned my brothers 'to leave me alone' or ' be nice'.
I am 35 OP, so not much younger than you, that you went through what you did, your Mother should be totally ashamed!
My 8-year-old dd and 10-year-old ds both know about periods, from when they were tots and walked in on me changing my pad, I explained that there was a house inside my tummy, a house that was built every month just in case a baby was to come to stay and that when no baby was there, the house came away onto my pad, so that next month my body could build a new house. I used very simple language, but they understood and when I explained periods and sex around age 6 (again in age-appropriate language) it was no great leap, all just normal and no embarrassment whatsoever.
My dh and I have since taken over teaching dd (me) and ds (dh), as well as having an open chat altogether. I am not well, dh does all the shopping, so dd being happy to ask dh to buy her pads is important. I have bought a very pretty box and filled it with different types/ flows/brands of sanitary towels, as well as the tiniest tampons I have ever seen (dd is showing signs of starting puberty early)! Both ds and dd have several age-appropriate books on puberty, growing up etc. I intend to buy nice comfortable underwear, chocolate (thanks for the idea Dad) and a nice soft hot water bottle to add to the box.