If it’s been four weeks and you haven’t been re-infected with another cold, it may be a sinus infection. If so, use a neti pot, eat spicy foods, do some aerobic exercise or at least significant amounts of walking, and take ibuprofen several times a day.
Also, try eating raw garlic and foods with a lot of ginger. Make sure to get enough sleep.
And give some tzedakah, say some tehillim and spend a few minutes doing teshuvah — perhaps the cold is Hashem’s way of getting your attention regarding a particular issue you need to correct?