Don't let the burn you feel in your abs after doing 100 sit ups or crunches fool you to thinking that it's burning belly fat because IT IS NOT. That is the oldest and most outdated misconception you can hold on to.
You need to execute more intense movements that involve bigger muscle groups in order to command your body to tap into fat reserves. In doing so, your body will then use your "overall body fat" as fuel, which then over time, lower your overall body fat percentage, which includes the fat in the belly area.
To give you an idea on how to effectively hit your ab muscles to maximize your result and effort, try doing this instead.
Spider climbers for 60 seconds or to failure, followed by crunches for 15-20 reps or until failure. Repeat 5 rounds. 30-60 second rest between sets.
Beginners, please modify the exercise as suggested in this video. When performing crunches, please DO NOT pull on your head/neck. Instead, focus on really contracting/flexing your entire core area as though you're about to get punched in the gut and you're trying to tighten it to brace for the punch.
Also, please ensure the middle of your back never leaves the ground. Press it against the ground during the crunch forward so there is no gap or space between your back and the ground.