Sit-ups are a total waste of time. Any time you’re doing any kind of exercise lying on the ground, it’s the laziest position your body can be in. You burn the least amount of energy and calories possible.
If you’re looking for flat abs, look at what you’re eating, because you could be bloated. You could have a lot of undigested food particles in your colon that are protruding in your stomach. Most women don’t go to the bathroom enough. A healthy person should be able to go to the bathroom a minimum of twice a day. If you’re not doing that, then you’re either eating foods that aren’t being digested or you have an issue with your colon.
You also want to do exercises that work a muscle called the transverse abdominis. It’s the deepest muscle fibre in your abdominal wall, the one deepest to your spine, and it’s the one that stabilizes your spine. It has to be activated against gravity, so doing exercises in an upright position with a tense abdominal area, like somebody’s about to punch you in the stomach, works better. Exercises like wood chops, planks, or even exercises like dead lifts and squats are fantastic for trimming the abdominals.