What should I do concerning the vows of obedience (such as prayer, fasting, and memorizing the Quran) that I vowed but forgot or am unable to fulfill?

If the vows you made were conditional upon a specific event occurring, and it has occurred, or if they were not conditional upon anything originally, then they have become binding upon you. What you remember and are able to fulfill, you are obligated to fulfill according to your capacity. As for what you are permanently unable to fulfill, or what you have forgotten, you should offer an expiation for an oath (kaffarat yamin) for each one, until you are reasonably certain that your obligation has been discharged. The prohibition of making vows has been mentioned, so do not embark upon it at all.