Those lines that I before have writ do lie,
Even those that said I could not love you dearer:
Yet then my judgment knew no reason why
My most full flame should afterwards burn clearer.
But reckoning Time, whose million'd accidents
Creep in 'twixt vows, and change decrees of kings,
Tan sacred beauty, blunt the sharp'st intents,
The speaker is now aware of how strongly he feels – the flame burns ‘clearer’ (and presumably hotter) than it did before. 
 He doesn’t suggest that his feelings for the boy were misplaced or in anyway wrong – but that he simply didn’t understand the depth of them until now.
Sonnet 115