If I add a new post, not created using cornerstone, then my last post that was created in cornerstone turns to the CS code. So then, if I update the newest post with cornerstone, the other post originally made with cornerstone also turns to the new text I just entered for the newest post! Please help!
You concern is a bit confusing. As far from what I understand, your cornerstone post turns into text post as well. Posts has its own entity. If the post is a cornerstone type it should load a cornerstone type of post. If it turns into text or something else, it should have been changed or updated to text.
Please check if you have the latest compatible version of X theme and Cornerstone.
If not, update your version to the latest version,
If still not helping, please share us your admin credentials so we could check your setup closer.
I can’t replicate this in my test site which means there is something specific in your set-up that is causing the issue.
Can you try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.