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Your save probably has a pawn trying to do a broken pawn. You can confirm this by turning on dev mode in settings, loading the save, then after it fails to load, open the debug console. You'll see an error there. Usually it's something related to a pawn doing a job incorrectly and the save fails to load. If you know how, you can graft an idle pawn's job onto the bugged out pawn by editing the save file, but if you aren't comfortable with doing that, try loading an earlier save instead.
If the error message isn't a pawn job problem, then I've no clue, sorry.

I don't think it has anything to do with a pawn job problem. The error says this:

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Exception from asynchronous event: System.ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
  at System.Collections.Generic.List`1[System.Int32].get_Item (Int32 index) [0x00000] in <filename unknown>:0
  at Verse.DefMap`2[RimWorld.WorkGiverDef,System.Int32].get_Item (RimWorld.WorkGiverDef def) [0x00000] in <filename unknown>:0
  at Fluffy_Tabs.PawnPrioritiesTracker.SetPriority (RimWorld.WorkGiverDef workgiver, Int32 priority, Int32 hour) [0x00000] in <filename unknown>:0
  at Fluffy_Tabs.PawnPrioritiesTracker.SetPriority (RimWorld.WorkGiverDef workgiver, Int32 priority) [0x00000] in <filename unknown>:0
  at Fluffy_Tabs.PawnPrioritiesTracker.SetPriority (Verse.WorkTypeDef worktype, Int32 priority) [0x00000] in <filename unknown>:0
  at Fluffy_Tabs.Detours_WorkSettings._SetPriority (Verse.WorkTypeDef w, Int32 priority) [0x00000] in <filename unknown>:0
  at RimWorld.Pawn_WorkSettings.Disable (Verse.WorkTypeDef w) [0x00000] in <filename unknown>:0
  at Hospitality.Detouring.Pawn_WorkSettings.CheckForDisabledTypes (RimWorld.Pawn_WorkSettings _this, Verse.Pawn pawn) [0x00000] in <filename unknown>:0
  at Hospitality.Detouring.Pawn_WorkSettings.ExposeData (RimWorld.Pawn_WorkSettings _this) [0x00000] in <filename unknown>:0
  at Verse.PostLoadInitter.DoAllPostLoadInits () [0x00000] in <filename unknown>:0
  at Verse.Game.LoadGame () [0x00000] in <filename unknown>:0
  at Verse.SavedGameLoader.LoadGameFromSaveFile (System.String fileName) [0x00000] in <filename unknown>:0
  at Verse.Root_Play.<Start>m__736 () [0x00000] in <filename unknown>:0
  at Verse.LongEventHandler.RunEventFromAnotherThread (System.Action action) [0x00000] in <filename unknown>:0
Verse.Log:Error(String)
Verse.LongEventHandler:RunEventFromAnotherThread(Action)
Verse.LongEventHandler:<UpdateCurrentAsynchronousEvent>m__733()
~~~~

At first it seemed like the mod was incompatible with fluffy's mods or Hospitality, so i disabled them to see if it would load Mending... and still no load.

Sorry if this is a noob question, but for some reason i can't seem to get the mod to load at all on a pre-existing save. Some previous posts have said that nothing is required at all to use the mod, besides to just place it in the proper folder. I have A16, and a few other mods on a previous save, however every time i load "mending" i get the "mods mismatch. this file was saved with a different set of mods than those we are currently running... etc" error screen. Doesn't matter where i put it in the order list. Other mods i have loaded with a previous save that were "plug and play" worked with no problem.