How thick is it now? If it's unusable I think the only thing to do is bin it, sorry. But don't forget there are several thicknesses of ca, including "jelly". That is useful sometimes, so your's may still be usable (it takes much longer to harden, though accelerator helps).
In future, keep all your ca in the fridge, that lengthens shelf life considerably
One way to keep it longer is to keep it in the fridge. It makes the usable life much longer. That's where mine has been kept for years now.
Slightly different, but still on adhesives. Epoxy will go very thick if it gets cold. To thin it just dunk it in hot water for a few minutes and it becomes the normal thickness again. But if it's 5 minute stuff and it's warm it cures a bit quicker too!
Not sure about diluting CA as I've never had to do that but will try mixing the thin and thick together, good call. In case anyone is interested......
I get a lot of my CA from Stewmac in the US for my guitar repairs. They also do Black, Amber and White CA and mixing them gives me variations of greys and creams. G Force models UK sell the super thin 'Zap' CA which is great for repairs that need glue to wick and set quickly, they also sell the accelerator. Buy whip tips and wear gloves as the water thin glue can be evil.
If you want the CA to last, put it in a fridge it really does improve it's longevity especially with the thicker varieties.
I had to do some repairs to the area next to the binding edge on an old guitar neck recently and no matter how many commercial fillers, and homemade fillers using wood dust/glues I tried, I couldn't get a match. Someone on t'internet suggested brown artist chalks packed into the area to be filled then dropping water thin CA glue on top, which was perfect. All of the above is probably useless info and a bit off topic but may be of help to someone...