Maybe go to a doctor and have a culture done -- viruses rarely hang around that long, if it is the same infection...
In the meantime, try your best to do the fun things mum told you to do when you get a cold

Get lots of sleep if you can, humidity is a good idea but the best way is to make sure you drink LOTS of water, as for taking things for it, try to avoid it if you can. Often they're more likely than anything else to just cause more complications.. it's kind of like gooshing on a toothpaste tube -- you make a dent but something else has to give. Relaxing and de-stressing and making sure you don't get chilled (because letting your body get too cold does weaken your immune system) will help.
If your doctor doesn't culture a bug, it's possible you might have a chronic inflammation. It's not likely, but your doctor will know best what to do in this situation.