Why does my dad tell me I'll die on a rollercoaster?

Because I USED to have epilepsy and I'm very skinny (size 8), my dad says my heart will stop or Ill slip out of the seat etc. Im going Thorpe Park with the school in two weeks and he keeps telling me not to go on the rides. What should I do? All my friends will go on them! (I'm 15 btw, thats how you know my dad's contolling.)