"My school days were the happiest days of my life; which should give you some indication of the misery I've endured over the past twenty-five years"

This quote by Paul Merton suggests that his life considering that his school days has actually been filled with suffering. He is suggesting that the happiest days of his life were during his school days, which implies that his life ever since has actually been filled with unhappiness and suffering. This could be due to a range of reasons, such as an absence of success in his career, a lack of meaningful relationships, or a lack of satisfaction in his life. It is possible that he is comparing his present life to the pleasure and innocence of his school days, and feeling a sense of remorse for not being able to recapture that very same sensation. Whatever the factor, it is clear that Paul Merton has experienced a lot of distress in the previous twenty-five years.