I remember one of the Radio 2 doctors saying "The important thing to remember about colds is that they never last longer than ten days. If your symptoms persist you either had a cold which has now given way to a secondary infection or it was never a cold to start with".
It's harvest season, so my guess is that you've got a form of hay fever. (I was in my twenties when I first got hay fever. My friend was in his sixties when he first had it, so you can get it at any age). While many people associate hay fever with just a lot of sneezing, those who've had it will tell you that it can be really debilitating, leaving you feeling like you've got 'flu.
As with any condition that doesn't clear up of its own accord within a reasonable time, it might be best to consult a doctor but, if you'd prefer not to trouble your GP, try asking your pharmacist for some loratadine tablets. (They're usually very cheap if you buy them under that generic name but ridiculously over-priced when branded as 'Claritin', which is exactly the same thing).