Sorry, but I have no sympathy. To not make a backup before installing a beta is just ridiculous.
Also to expect a previous version to somehow magically open a newer version is also ridiculous.
To convert back to a previuos version is very easy.
This will only work as long as you haven't made use of any new features, obviously.
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, r214 will look like 21400). Change the first digits to match version you want to revert to, i.e. 212.2 would be r21202, 21200 would be r212.
7. Double click .caproj to open within C2