Is what I am doing—designating specific clothes for prayer and changing my undergarments before each prayer—correct due to the excessive doubt I experience regarding the cleanliness of my clothes?

It is advised to disregard doubts and act naturally, without changing clothes for every prayer. Pray in your clothes unless you are certain they have become impure, and build upon the principle of purity, because certainty is not removed except by a similar certainty. This repels affectation, extremism, and exaggeration, and closes the door to obsessive whispers.