Thanks Leocg! You're help is awesome. I bumbled my way through it before seeing your post, though.
I'm writing down the instructions of what I did in case anyone else is having this problem. (I'm using Windows 10, btw.)
Re-installing the same (newest) version of Opera resulted in the same problems. I had to go with the Beta version, which works great. However, the bookmarks were NOT saved when I downloaded the Beta version.
I was able to salvage my bookmarks this way.
BEFORE uninstalling Opera
(1) If you have bookmarks in your unsorted bookmarks folder, transfer them into a new folder.
(Why? Because Opera Beta does not have an unsorted bookmarks folder. You will not be able to find those bookmarks on Opera Beta. However, when you go to the websites, they will have a heart as if they are saved, but you won't be able to delete the heart so that you can re-add it. It will create a problem where not only are these bookmarks lost, but you can't even re-add them)
(2) Go to this file location:
C:\Users\my username\AppData\Roaming\Opera Software\Opera Stable
The part that says "my username" you will have to fill in with whatever your username is.
Or you might be able to find it here
C:\Users\AppData\Roaming\Opera Software\Opera Stable
(3) There are two files called "Bookmarks" and another called BookmarksExtras. Also a file called Shortcuts and Shortcuts-journal. Copy them into a separate folder on your desktop.
(4) Then you can uninstall Opera
When uninstalling Opera, DON'T tell it to forget your user data, just in case.
AFTER uninstalling Opera
(5) Install the Beta version.
(6) Now go to this file location
C:\Users\my username\AppData\Roaming\Opera Software\Opera Next
If that doesn't work, just go to
C:\Users\my username\AppData\Roaming\Opera Software
and go into the other Opera file (besides Opera stable)
(7) Now go to the folder where you saved copies of the bookmarks.
Copy them into the Opera Next folder.
Make sure that Opera Beta browser is closed when you do this.
(8) Now open Opera Beta browser, and your bookmarks should be there!