Chronic Sinusitis: Given the patient's history of sinusitis and symptoms of sinus pressure, headache, and yellow discharge lasting for 2 weeks, chronic sinusitis is the most likely diagnosis. The prolonged duration of symptoms and the specific nature of the discharge point towards a chronic infection or inflammation of the sinuses.

Acute Bacterial Sinusitis: This condition presents with similar symptoms, including sinus pressure, headache, and purulent nasal discharge. The key differentiator is the duration and severity of symptoms, but it's a plausible diagnosis given the patient's history.
Allergic Rhinitis: Although typically associated with clear nasal discharge, in some cases, especially if there's a secondary infection, the discharge can become yellow or greenish. The headache and sinus pressure can also be part of the symptomatology.
Viral Upper Respiratory Infection: While often presenting with clear discharge, some viral infections can lead to secondary bacterial infections, resulting in yellow discharge. The headache and sinus pressure can be part of the viral infection's symptom complex.

Meningitis: Although rare, meningitis can present with headache and, in some cases, sinusitis symptoms if the infection spreads to the meninges. It's crucial to consider this diagnosis due to its high morbidity and mortality if not promptly treated.
Orbital Cellulitis: An infection that can arise from sinusitis, especially if the infection spreads to the orbit. Symptoms can include headache, sinus pressure, and changes in vision, making it a critical diagnosis not to miss.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening complication of sinusitis, where a blood clot forms in the cavernous sinus. It requires immediate medical attention.

Fungal Sinusitis: More common in immunocompromised patients, fungal sinusitis can present with similar symptoms to bacterial sinusitis but may have a more insidious onset and different treatment approach.
Tumor (e.g., Nasopharyngeal Carcinoma): Although extremely rare, a tumor in the nasal or sinus area can cause obstructive symptoms leading to sinusitis, headache, and discharge. It's essential to consider in the differential, especially if symptoms persist despite appropriate treatment.
Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can affect the sinuses, among other areas, and present with a variety of symptoms including sinusitis, headache, and discharge.