I'm not familiar with how Mint manages your drivers, but now that you know the solution, I just want to point out a thing:
It's good you came up with the solution by yourself, because this is a must for using Linux distros, especially for mint, which inherited the userfriendliness of Ubuntu, and the hackability of the allmighty Debian
Sometimes if you use the bleeding edge you run on problems that needs fixing, and you have to do it all by yourself, by actually using your BRAIN. Congratulations, this is a rare deed nowadays
Now, to save you from further frustrations on kernel upgrades and proprietary video drivers:
1st rule of thumb: you only upgrade the kernel if it's a MUST. If it ain't broken, don't try to fix it! (Your case now was a must, so you were right on that).
2nd: If you upgrade the kernel, you always have to rebuild the proprietary drivers, because those are not part of the kernel package by default. You don't have to compile it from source, some ditros ship them as precompile binary packages, but then you have to install the right package for the right kernel.
3rd: to save you from this manual work nonsense, you might try to use DKMS. If you don't know what that is, google it! I'm no sure if mint supports it, but on Debian it helps a lot (if it works, haha).
"One should strive to achieve; not sit in bitter regret."