Tired of Body Filler Hardening Too Quickly?

Had my filler, hardener in the fridge for the night. After mixing up at 68F, it took exactly 4m30s for curing. At 87F it took 90 seconds.
I cant work with that kind of product. Is my filler a defective product? What am I doing wrong? Is there not some kind of chemical I can add to the mix to slow down the curing?