There is probably nothing very serious that is the matter with you, but this does not mean that you should continue to suffer from these symptoms! I strongly recommend that you go to see a primary care doctor
, because based on your description of your symptoms I am confident that there should be a simple, inexpensive, and effective solution to your problems.
Most likely, you are suffering from either nasal allergies or from chronic sinus congestion. Both of these conditions would explain your chronic nasal stuffiness and sensation of mucus draining down the back of your throat. Your primary care doctor can confirm the diagnosis usually just with a simple physical examination without the need for additional testing.
If confirmed, then the treatment is usually straightforward. It includes anti histamine medications and/or nasal steroid sprays to reduce the swelling in your nose, open up your sinus passages, and promote proper drainage of secretions. The medications are safe and inexpensive and rarely have side effects. I suspect that, with this type of treatment, you will be feeling a lot better very soon.
Please make an appointment with your primary care doctor to have this problem checked out at your earliest convenience.