Gastroparesis: This condition is characterized by delayed stomach emptying and is a common complication of long-standing diabetes. The symptoms of persistent diarrhea, indigestion, and the feeling of the stomach bursting, especially in the late evening, align with gastroparesis, as it can cause nausea, vomiting, bloating, and abdominal pain due to the slowed movement of food through the stomach.

Diabetic Enteropathy: This condition involves damage to the nerves of the intestines (enteric nerves) due to diabetes, leading to gastrointestinal symptoms such as diarrhea, constipation, and abdominal pain. The long duration of symptoms and the presence of uncontrolled diabetes make this a plausible diagnosis.
Small Intestine Bacterial Overgrowth (SIBO): SIBO can occur in patients with diabetes due to altered gut motility. It leads to symptoms like bloating, abdominal pain, diarrhea, and indigestion, which match the patient's complaints.
Celiac Disease: Although less common, celiac disease can be associated with diabetes and causes gastrointestinal symptoms similar to those described, including diarrhea and abdominal discomfort.

Gastrointestinal Cancer: Although less likely, gastrointestinal cancers (e.g., stomach or pancreatic cancer) can present with nonspecific symptoms like indigestion, abdominal discomfort, and changes in bowel habits. Given the potential severity of these conditions, they should not be overlooked.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic diarrhea, abdominal pain, and weight loss. While the primary symptoms might not directly suggest IBD, the chronic nature of the symptoms warrants consideration.
Pancreatic Insufficiency: This condition, which can be secondary to diabetes, involves the pancreas not producing enough digestive enzymes, leading to malabsorption and symptoms like diarrhea and weight loss.

Amyloidosis: A rare condition where abnormal proteins (amyloid) accumulate in various organs, including the gastrointestinal tract, leading to symptoms like diarrhea, weight loss, and abdominal pain. Its association with diabetes and the chronic nature of the symptoms make it a rare but possible consideration.
Chronic Intestinal Pseudo-obstruction: A rare disorder characterized by impairment of the muscle contractions that move food, fluid, and air through the intestines, leading to symptoms like severe abdominal pain, nausea, vomiting, and altered bowel habits.