I’m sorry to tell you things you probably don’t want to hear. What you describe as “controlling” is, from your grandmother’s point of view, being a good parent. If you want to live with her, you need to respect her desire to see you study hard and make good grades and you need to live by the rules of your grandparents’ house.
Going “on strike” by taking to your bed and insisting on a sexual relationship that is against her values is not being the mature person you claim to be. If you were responsible about your schooling and acted more grateful that she took you in, you might be able to negotiate a more adult relationship. As it is, you are showing her that her concerns about you are justified.
At 18, you have other choices than living with your grandmother. If you don’t like her rules, go back to your mother’s house or get a job and live on your own. You are not going to change your grandmother so it’s up to you to change the situation — either by leaving or by accepting that you give up a certain amount of control when you are living with someone at her tolerance. I hope you can make an adult decision about what is best for you at this point in your life.
I wish you well.