Nasal congestion that has been ongoing for this long may be do to an allergy that you do not know you have. It is less likely a sinus infection or sinusitis because these types of infections cause other symptoms and would not last this long. Determining what you are allergic to can be tough, and you may warrant a referral to an allergist for allergy testing.
If your symptoms do not seem to be linked to any particular time of year, then you are likely allergic to something in your house. One common allergy is an allergy to dust mites. Patients with dust mite allergies tend to have stuffy noses year round, and get worse when they spend time in dusty rooms. You may find that your symptoms are worse the day you clean, and better in the days following.
I think there are a couple of acceptable routes you could take. You could try an over-the-counter antihistamine and see if this helps your symptoms. Please consult your primary care physician
before starting this or any medicine. If the anti-histamine does help, then you will know that you have an allergy. From there you can schedule an appointment with an allergist who can further work-up your symptoms. Good luck.