Firstly, to save over your work with a new beta release without first making a backup is unforgivable.
Having said that, it is easy to revert to a previous version by simply editing one file:
This will only work as long as you haven't made use of any new features after r212.2, obviously.
Summary:
1. If it's a project file - a .caproj file, skip step 2.
2. If it's a capx, unzip it (capx are just zip files basically) using such as Winzip, 7 zip etc, or load and save as a Project.
3. Now, open the caproj file in a plain text editor.
4. Look for the following string, minus quotes: "<saved-with-version>". It's only a few lines down from the top.
5. You'll see this tag has a 5 digit number; it's the c2 version padded with 2 zeros (so, r213 will look like 21300). Change the first digits to match version you want to revert to, i.e. 212.2 would be 21202.
6. Save
7. Double click .caproj to open within C2