What is the ruling on frequent doubt and forgetfulness in prayer, such as doubt about not bowing (ruku') or reciting the tashahhud, and is it obligatory to repeat the prayer in this case?

The best cure for doubts is to ignore them. Whoever experiences frequent doubts, to the extent that they occur daily, even once, should ignore them and not act upon what they doubt. No prostration of forgetfulness (sujud sahwi) is required of them. If you are prone to frequent doubts, do not perform what you suspect you omitted, whether it be the Fatiha, bowing (ruku'), or the tashahhud. It is not prescribed for you to repeat the prayer or the pillars that you doubt having omitted.