Discredited theory, FYI. Autism and wheat/milk have no connection.
Stimming is not something you should be trying to stop. It's healthy and normal for everyone, not just autistic people; and we do it for many reasons, not least of which are that it helps us think more clearly, regulate sensory input, reduce stress, and compensate for dyspraxia/sensory hyposensitivity. OP, your parents probably don't know why you stim, and are only trying to make you stop because it is unusual.
If you want to stop a stim, you need to replace it with another that serves the same purpose. This is "redirecting" a stim, and it's a much better strategy than just trying to stop. You should try to replace one stim with another if it is something that hurts you or something that causes problems for other people, such as being very distracting in a classroom environment. There are other solutions than changing the stim, though; for example, if your rocking is distracting classmates and you can move to the back of the room, you may not have to change it.