What should I do regarding vows I made to Allah at the age of fifteen, such as praying four rak'ahs of Sunnah before Asr and refraining from daydreaming, if I have sometimes failed to fulfill them, especially when time is short or there is a need to catch the obligatory prayer or congregational prayer?

If the promise to pray was merely an internal thought and not uttered verbally, then nothing is binding from it. However, if you did utter it verbally, then you are not obligated to fulfill it; rather, it is only recommended. Consequently, there is no make-up for those prayers and no expiation. And no expiation is required from you for the other promise either.