She bid me take love easy, as the leaves grow on the tree; (5-6)
Advice is always easier to take after having experienced the consequences of not taking that advice. The speaker is in the same boat here, looking toward the past and realizing that his love had some good advice here that he just wasn't sharp enough to take.
But I, being young and foolish, with her would not agree. (7-8)
Oh, foolish young boys just refuse to listen to anyone's advice and the speaker is no different here. Not taking good advice is kind of part of being young and foolish and needing to learn from your own mistakes.
But I was young and foolish, and now am full of tears. (15-16)
What's the consequence of looking to the past and realizing you had been young and foolish? You end up full of tears, of course. But don't worry, it's all totally normal and it also seems as if memory is there to help provide a little insight and a few tough lessons.