I'm not going to let my girlfriend follow my instagram
Why don't you want your girlfriend to follow your instagram?
Because our relationship is dead.
Trust is a huge part of relationships. If you feel like your relationship is over, you need to talk to your girlfriend rather than telling her what not to do.
Why do I have to trust her, I did nothing wrong.
I am sorry that you are having a hard time trusting her and maybe you should end things with her. It is better than expecting her to stop doing what she wants.