Should I add a rak'ah, say the taslim, and perform the prostrations of forgetfulness if someone tells me that I prayed one rak'ah instead of two, given my frequent doubts and tendency to build upon an increase in my prayer?

If doubt assails you every day, even once, then it is a pervasive doubt that should be disregarded. You should build upon the most certain, and nothing is required of you, but try to achieve humility. However, if a trustworthy person informs you of a deficiency in the number of rak'ahs, then you are required to complete the missing rak'ahs and then perform the two prostrations of forgetfulness (sujud as-sahw), because the doubt has been dispelled by their information.