What should one who frequently lapses in prayer and has misgivings about everything, even the Tashahhud, do, especially when he remembers reciting the Fatihah and prostrating but does not remember bowing? And what advice is there for him?

As long as you are afflicted with obsessive whispers (waswasa), turn away from these whispers and do not pay any attention to them. So, no matter how much you doubt whether you omitted recitation, bowing (rukūʿ), or prostration (sujūd), do not pay attention to this doubt and proceed with your prayer. Prostration of forgetfulness (sujūd as-sahw) is not required of you. However, if you are certain that you omitted a pillar (rukn) of the prayer, then you must perform it and whatever comes after it. If you have already begun the next prostration, then you invalidate the one in which you forgot a pillar and make the subsequent one take its place. Then, you prostrate for forgetfulness at the end of your prayer. This applies only with absolute certainty, to the extent that you could swear that you omitted a pillar of the prayer. As for mere doubt, you should do as we mentioned: turn away from the whispers.