Is it necessary to repeat prayers that were performed while wearing underwear that had yellowing and stains on it after being washed, knowing that there are obsessive thoughts concerning its purity?

Waswasah (obsessive whispering/doubts) is a dangerous disease that corrupts one's religion and worldly life. Its cure lies in turning away from it, not paying attention to it, and not yielding to it. Your prayer in the clothes mentioned is valid, and you are not required to repeat it. The mere presence of stains does not invalidate the prayer unless you are certain that they are impurities. Your prayer is valid if you prayed in an impure garment out of ignorance of the impurity's presence or inability to remove it, as avoiding impurity is a condition when one has knowledge and ability.