Discontentment
The central theme of this poem is that of discontentment, and the poet projects this quality onto the reader from the very beginning. The very title shows that this poem is going to be talking about some sort of regret. As a narrator, the poet is essentially unhappy that he has given up his wisdom by succumbing to worldly pleasures of passion.
Spirituality/Morality
Another theme that is latent in this poem is that of spirituality, or rather, morality. The poet seems to be reflecting about his actions and wondering whether it was right to succumb to the lure of materialistic pleasures. There is a strict sense of moralism in this poem, an idea that is propagated through words like "austere" and "ancient wisdom".
Passions
The source of his discontent is what the poet believes to be a weakness in him. He believes that the fact that he indulged in passions means that he has lost his wisdom and control. Passions play a negative role in this poem and are the source of the poet's angst.
