Is my prayer valid, or should I repeat it, given that I prayed and forgot to change my underwear, which I suspected was contaminated with urine?

You do not need to repeat [the prayer] as long as you are not certain that urine has exited, because the default is that it has not exited until that is confirmed. Certainty is not removed by doubt. If you are certain of your purity and doubt the occurrence of impurity, then the ruling is that you remain in a state of purity. If we assume that your clothes were impure and you forgot to change or purify them until you finished the prayer, then the prayer is valid, according to the preponderant opinion.