What is the remedy for obsessive doubts (waswas) concerning ablution (wudu') when there is a recurring feeling of passing gas that reaches the degree of certainty, and how can one distinguish between obsessive doubt and certainty in this situation, along with the fear of the prayer not being valid if ablution is not repeated?

If the questioner is not certain about the occurrence of the impurity (by hearing a sound or smelling a scent), then what she perceives is merely a doubt, and ablution (wudu') is not nullified by it. The cure for doubt is to disregard it. However, if she is certain about the emission of wind by hearing a sound or smelling a scent, then ablution is nullified. This applies if its emission is not continuous. If its emission is continuous, then the rulings pertaining to سلس (incontinence/continuous discharge) apply to her.