We've said it before, but we'll say it again: any time you start a piece of literature with a reference to a wise man, chances are that the wise man will be ignored and some really stupid mistake is coming around the corner. "When I was One-and-Twenty" is no exception. Unless, of course, you happen to disagree with the "wise man" when he says that love is a game that only fools play. After all, falling in love may be foolish – but, as this poem demonstrates, falling away from love might be just as ridiculous.
The wise man in this poem isn't actually wise. It's the speaker who has it right from the beginning.
The speaker's first mistake was to start thinking of his heart as something that could be exchanged for other stuff.