My Achievements frequently 'forget' that I've accomplished any of them and reset as if I've never played before. Why does this happen, and is there some kind of fix for it?
I'm afraid it's just a bug:
Achievements also reset when you update the game, install mods, uninstall mods, etc.
there is one way i found that can be use to get your stats from the previous session...
this works effectively if you start this from the point you create a new world
step 1> start playing your world in 'minecraft'
step 2> as soon as you enter, press pause and quit (Don't quit the game entirely, just get back to the main menu.)
step 3> come out of the game and open the folder ..\.minecraft\saves\**world name**\stats
setp 4> find the file that was most recently created ex:filename-some random values.json (you can confirm this by checking the modified time of the file)
step 5> now find the file that was created when you played the previous session (If you follow this technique from the start, there would only be two files in the folder making it easier to find out which one was created during the current session and the previous session)
step 6> open the previous session file in notepad and copy all its contents
step 7> open the current session file in notepad and overwrite any information present
step 8> save the current session file and delete the previous session file
step 9> get back to your game and enjoy your world
Hopefully this will help until a fix comes out for this problem
Replace your 1.7.10 launcher (delete the contents of minecraft launcher folder) with the 1.6.4 launcher. If you have a copy of that saved somewhere on your PC, copy the contents of the Minecraft launcher folder of 1.6.4 and paste it in 1.7.10 (choose your profile and then play), I had the problem but I did this and now it's okay.