WARNING: When uninstalling Frostfall, the following (simple) procedure must be followed, or permanent damage to your character will result that you will most likely be unable to fix.
Step 1: Back up your save games, just in case. Don't ignore this, it's important!
(Optional Step): If you have anything stored in a chest from the Conjure Shelter spells, conjure a shelter and remove those items from the chest now. If you do not do this, the contents of the chest will be lost permanently.
Step 2: Go inside a building. It doesn't matter where.
Step 3: Wait 30 seconds of real time OR wait until the "Frostbitten" effect has worn off.
Step 5: After exiting all of the menus, wait another 30 seconds of real time.
Step 6: Save your game.
Step 7: Disable or Uninstall Frostfall using your Mod Manager (such as the Nexus Mod Manager). Also, uncheck any other mods that directly rely on Frostfall (ex: Knapsacks Enhanced). Failure to do this will result in a crash to desktop directly after the Bethesda logo on boot-up.
Load your game and ensure that all of your skills are set to their default values. If they are not, ensure that you do not have another mod that is affecting your skills at this time as well. If everything in your skills menu looks correct, Frostfall has been correctly shut down and uninstalled.
If you followed all of the instructions above to the letter, your game and your character should be fine. If not, chances are overwhelmingly high that you did something wrong, or you're using another mod that is also affecting your skills. I know this because I've tested it! Here are the test results.
Test Load Order:
After backing up my save games (Step 1), I loaded a game save that has never "seen" Frostfall before. In that regard, this is like fresh-installing Frostfall into an existing savegame. I then started the mod by looking to the stars. I then traveled to Winterhold, made a campfire, and pitched a tent. I also got myself very cold, and very wet.
Click images to expand.
I then checked my skills menu, and confirm that all of my skills are reduced as expected.
I then travelled to the Bannered Mare in Whiterun, as per Step 2. There, I waited 30 seconds (Step 3) and shut down the mod using the SkyUI Mod Configuration Menu (Step 4). I then waited 30 real seconds (Step 5) and saved my game (Step 6). I then quit to desktop.
Here is the Papyrus log I captured of this entire procedure as I was playing.
I used the Nexus Mod Manager to deactivate and uninstall Frostfall (Step 7).
I then loaded my save game and verified that my character had his skills restored to their proper values (Step 7, continued).
I then walked from Whiterun to Riverwood, waited 12 hours, killed a wolf, and made Sven angry by completing Faendal's quest. This constituted about 6 minutes of play time.
Here is the Papyrus log I captured after loading my game after Frostfall was uninstalled. Notice the errors that read:
These errors are expected and will occur at the top of your log file each time you load this save game from this point forward. They are safe to ignore. Also notice that after roughly 6 minutes of gameplay (as evidenced by the timestamps), you will notice that no log errors occur that point back to Frostfall.
TIP: If the log error doesn't include the prefix "_DE_" in the error, it does not originate from Frostfall.
You can also use these example Papyrus logs to reference how many, and what kinds, of log errors are generated from the base game alone (there are a lot!)
At this point, I consider uninstalling Frostfall a very safe, and very tested, thing to do.
If you believe that your character is still permanently affected in some way, even after following the above instructions exactly, you need to provide strong evidence that this is true by sending me the following to my email address, email@example.com...
...or else, I will not believe you, and even if I did, I will not have enough information to investigate the problem. Sorry if this is a bit harsh, but every single person's computer and Skyrim installation is different, and without this information, it's a lot like trying to fix a car while blindfolded.