Irritable Bowel Syndrome (IBS): The patient's symptoms of long-standing abdominal pain in the left upper quadrant (LUQ), bloating, and intermittent pain that occasionally resolves with peppermint oil are classic for IBS. The migration of pain to the back in the mornings could be related to bowel habits or referred pain, which is also consistent with IBS.

Gastroesophageal Reflux Disease (GERD): The patient's symptoms of abdominal pain and bloating could be related to GERD, especially if the pain worsens after eating or at night. The pain radiating to the back is also consistent with GERD.
Functional Dyspepsia: This condition is characterized by recurrent or persistent upper abdominal pain or discomfort that is not exclusively relieved by defecation or associated with a change in stool frequency or form. The patient's symptoms of intermittent pain and bloating are consistent with functional dyspepsia.
Inflammatory Bowel Disease (IBD): Although less likely given the long-standing nature of the symptoms and the lack of other systemic symptoms, IBD (e.g., Crohn's disease or ulcerative colitis) could present with abdominal pain, bloating, and changes in bowel habits.

Pancreatic Cancer: Although rare, pancreatic cancer can present with abdominal pain radiating to the back, and it is essential to consider this diagnosis, especially in older adults or those with risk factors (e.g., family history, smoking).
Peptic Ulcer Disease: A peptic ulcer can cause abdominal pain that radiates to the back and can be exacerbated by food or relieved by antacids. Although the patient's symptoms have been long-standing, a peptic ulcer should be considered, especially if there are risk factors (e.g., NSAID use, Helicobacter pylori infection).
Gallbladder Disease: Gallstones or cholecystitis can cause abdominal pain that radiates to the back, typically in the right upper quadrant but can occasionally be referred to the left side.

Intestinal Obstruction: Although unlikely given the chronic nature of the symptoms, an intermittent intestinal obstruction could cause abdominal pain, bloating, and changes in bowel habits.
Celiac Disease: This autoimmune disorder can cause abdominal pain, bloating, and changes in bowel habits, especially after consuming gluten-containing foods.
Gastroparesis: This condition is characterized by delayed gastric emptying and can cause abdominal pain, bloating, and nausea, especially after eating.