Then stop sharing her. Like, this seems pretty simple to me. If you agreed to an open relationship at the beginning and now you’re not okay with it anymore, that’s completely valid. People are allowed to change their minds and you don’t have to keep doing something that’s making you miserable.
You need to tell her how you feel and that you want the arrangement to change. She can either agree to be exclusive with you or she can’t. If she can’t, then you have to decide if this relationship is worth the pain of sharing her. Because staying in a situation you hate while pretending you’re fine with it is going to destroy you and probably the relationship too.
Don’t be ashamed of wanting exclusivity. Even in 2024, it’s still the most common relationship style for a reason. Wanting your partner to only be with you doesn’t make you jealous or controlling or insecure. It makes you normal. There’s no shame in that.
If she tries to make you feel bad for changing the rules, remember that relationships are supposed to work for both people, not just one. You’ve been accommodating her needs and now it’s time for your needs to matter too. Speak up before resentment builds up so much that it kills the relationship entirely.