Stress definitely matters, but honestly you're probably just not eating right (not your fault, most diet advice is rubbish). Type-2 diabetes is mostly about insulin resistance, and you can become insulin resistant without necessarily being fat just by eating things that provoke a strong insulin response. All foods have an insulin response, but generally speaking, from high to low: sugar, refined carbohydrates, unrefined carbohydrates, protein, fat. If you limit things that have a high insulin response you'll probably improve!