What should I do concerning the prayers in which I doubted having forgotten a pillar, and then completed the prayer and performed an additional rak'ah after the tashahhud, or canceled the rak'ah and performed another in its place, knowing that this doubt occurred frequently and I now suffer from obsessive doubts (waswas) in prayer?

The authoritative opinion among us is that whoever doubts having omitted a pillar is like one who has omitted it. If one doubts omitting it from one rak'ah (unit of prayer) and has reached its place in the subsequent rak'ah, then that rak'ah is nullified, and the second rak'ah takes its place. If one has not yet reached its place, then one returns and performs it.

This applies if one is not afflicted with waswasa (obsessive whispering/doubts). As for the one afflicted with waswasa, they should ignore these doubts and assume that they have performed what they doubted, based on the saying of Ibn Uthaymeen: "Doubt after performing an act has no effect, and similarly when doubts become numerous." Your previous prayers are valid, as the one afflicted with waswasa is permitted to adopt the easiest of opinions to alleviate hardship.