I have had a bad cold for around six weeks now. At first I just had a runny nose and cough, but now I have a headache and feel tired.?

Sinus infection. About 10% of the patients with an upper respiratory viral infection end up with a sinus infection. See your doctor.
"bad cold" Colds will only last 5-7 days. Since your symptoms are much longer, you likely have developed bacterial sinusitis. You need to see your primary physician for evaluation. Over-the-counter meds may help in the meantime. Afrin nasal spray used only 3-4 days can especially improve sinusitis symptoms.