How do I overcome obsessive doubts and uncertainty in acts of worship, and what is the ruling on doubting an act of worship after completing it?

It is advisable to turn away from whisperings (waswas) and ignore them, not paying them any heed, because indulging in them leads to great evil. Allah does not hold one accountable except for what He has legislated, and among His legislations is turning away from whisperings. If you desire Allah's pleasure, then abandon them, and know that they are from Satan, and ignoring them is pleasing to Allah the Almighty.