It needs to be warmer, can you do this in a heated basement or something.. it needs to be like 70 (at least) for it to harden correctly... :-\
I'm not sure if you know what the finished product should feel like.. but it should feel like "plastic" remember those plastic army men ?? It should feel like that...
If your bondo was stored outside.. and was cold before it was hardened that also might have ruined it, maybe try gettin a new canister of bondo (if you can afford it)