Is it obligatory to make up prayers that were missed intermittently over 20 years of discontinuity, knowing that the questioner has been consistent in prayer for the past 15 years?

Prayer is a great pillar of Islam, and whoever neglects it will be disappointed and lost, and must hasten to repent. Most scholars hold the view that all missed prayers must be made up. If you cannot ascertain their number, continue making them up until you are reasonably certain that you have fulfilled your obligation. Some scholars, like Ibn Taymiyyah, are of the opinion that making them up is not obligatory. You must make up prayers every day according to your capacity without causing yourself harm.