Most of the time it is diet... sometimes its a little bit of posture.
To fix your diet problems, keep your calories at the same level but get rid of sugar and cut your carbs by half. If that doesn't work, cut them farther. My long term advice would be only to get carbs from veggies and some fruit but no more than once every other day on the fruit and in the morning.
For posture, I really don't like to recommend sit-ups. Try the PLANK (Youtube Video)
several times a day. At first I would start with doing it for 3 - 5 minutes of total time throughout the day. Adding in some other resistance and free weight exercises would probably be very helpful.
If you goal is to loose the tummy without loosing much weight, cardio is not your answer.