The poem
A THOUGHT ay like a flower upon mine heart, And drew around it other thoughts like bees For multitude and thirst of sweetnesses; Whereat rejoicing, I desired the art Of the Greek whistler, who to wharf and mart Could lure those insect swarms from orange-trees That I might hive with me such thoughts and please My soul so, always. foolish counterpart Of a weak man's vain wishes ! While I spoke, The thought I called a flower grew nettle-rough The thoughts, called bees, stung me to festering: Oh, entertain (cried Reason as she woke) Your best and gladdest thoughts but long enough, And they will all prove sad enough to sting !

The thought that came to me is that we do not manage our self confidence and that our beginning thoughts about ourselves may be positive but as we let our doubts creep in, our thoughts turn from flowers that are lovely to smell to bees that sting.
Actually when looking further onto it, it seems to me to be about how when at first we think about memories or thoughts which make us happy, they seem so great and amazing. yet the more we remain in this thought, the more we realise that even such a moment of pleasure, was filled with much pain... A sort of cynicism..but that's just me I suppose.