Do i need to push myself to muscle fatigue everytime i do ab exercises to see results?

No: You don't have to do that. A variety of abdominal strengthening exercises at a moderate number of reps is sufficient. Easier ones, you can do more reps, but challenging one's, such as scissoring, you may have to do less. Of course, remember that diet and weight control are just as important if it's well defined abdominal muscles you are looking for.