Every time I make jam I always manage to set it too hard. Is there any way to recover it once it's done this?
The only way I know is by adding some more liquid and warming the mixture, It may or may not improve things...
For what its worth, the consistency of the jam after cooking, tends to be related to the amount of sugar added for a given fruit, with hard skinned fruit typically requiring a slightly different process to soft skinned fruit. It's also controlled by the amount of pectin added to the brew as well as the overall cooking time and temperature.
With my extremely limited knowledge of jamming, I would guess you could try cooking up some more of the fruit, no sugar, no pectin, and stirring a little bit in at a time. As Pulse said, it's the sugar / pectin ratio that get you 'jam'.
I'm extrapolating from my meager knowledge to think that you could lower that ratio with more fruit added.
I'm going to be jamming up some berries soon; if I have a chance, I'll post results here.