As Froggy says, get well soon.
As for your tin of filler, maybe it's just me, but I read that data to mean it was manufactured in 2011 and was 'best before' April 2016, so it's possibly a year out of date.
There are other factors that could affect the malleability and setting time though - cold weather will make the filler stiff while the amount of hardener you add dramatically alters the setting time.
When it's cold outside, consider keeping your tin of filler near a radiator indoors to keep it soft. Also try mixing in only about a third of the usual amount of hardener when you're going to fill large areas. That will keep it workable long enough for you to smooth it off with a spreader instead of it going stiff half way through and turning into a diorama of the Himalayas.
The only downside with that is that you'll need to leave it a lot longer before it does set and I find it stays tacky for about eight hours if it's mixed to be workable for ten minutes. I just leave large areas for the last job of the day and allow it time to set fully overnight.
As for which filler to use, I've tried loads of different ones that all use terms like 'easy sand', 'flexible', 'lightweight' etc., and they all seem to be exactly the same, so I buy what's cheap at the time.
This appears to be the cheapest at the mo' (unless anyone can suggest a cheaper alternative?) with the added bonus of them including ten spreaders -
http://www.ebay.co.uk/itm/2910667433...%3AMEBIDX%3AIT
Anyway, good luck with the prep and paint, and can you just remind me which colour you're going with as I've forgotten.
*just looked at that tin again, I now see it was filled at 08:02hrs on 11th April 2016 and presumably has a shelf life of two years, so it's probably the weather/catalyst that's making it difficult to work with.