Gastroesophageal Reflux Disease (GERD): This is the most likely diagnosis given the nature of the pain (non-radiating substernal chest pain) and its duration (two months). GERD is a common condition that can cause chronic or recurrent chest pain due to the reflux of stomach acid into the esophagus.

Esophageal Spasm: This condition involves abnormal muscle contractions of the esophagus, which can cause chest pain that is often described as squeezing or tightening. The pain can be substernal and may not radiate.
Peptic Ulcer Disease: Although peptic ulcers typically cause abdominal pain, they can also cause chest pain if the ulcer is located near the gastroesophageal junction. The pain can be substernal and may worsen with eating or lying down.
Pericarditis: Inflammation of the pericardium, the sac surrounding the heart, can cause sharp, stabbing chest pain that is often substernal and may improve with sitting up and leaning forward.

Acute Coronary Syndrome (ACS): Although the pain is described as non-radiating and has been present for two months, it is crucial not to miss ACS, which includes conditions such as myocardial infarction (heart attack) and unstable angina. These conditions can present atypically, especially in diabetic patients or women, and can have a subtle onset.
Pulmonary Embolism (PE): A blood clot in the lungs can cause sudden onset of chest pain, which may not always radiate. Although less likely given the chronic nature of the pain, PE is a life-threatening condition that must be considered, especially if there are risk factors such as recent travel, immobility, or family history of clotting disorders.
Aortic Dissection: A tear in the inner layer of the aorta can cause severe, tearing chest pain that may not radiate. This is a medical emergency that requires immediate attention.

Esophageal Cancer: Although rare, esophageal cancer can cause chronic chest pain, difficulty swallowing, and weight loss. The pain can be substernal and may not radiate.
Mediastinal Masses: Abnormal growths in the mediastinum (the area between the lungs) can cause chest pain, among other symptoms, depending on what structures they compress or invade.
Costochondritis: Inflammation of the cartilage that connects the ribs to the breastbone can cause sharp pains in the chest, which may be substernal and can mimic other conditions.