There is a change--and I am poor;
Your love hath been, nor long ago,
A fountain at my fond heart's door,
Whose only business was to flow;
And flow it did; not taking heed
Of its own bounty, or my need.
What happy moments did I count!
Blest was I then all bliss above!
Now, for that consecrated fount
Of murmuring, sparkling, living love,
What have I? Shall I dare to tell?
A comfortless and hidden well.
A well of love--it may be deep--
I trust it is,--and never dry:
What matter? If the waters sleep
In silence and obscurity.
This contrasts the original  metaphor  for their love which was a ‘fountain’. A fountain suggests an overflow of positive emotions.  A ‘well’ is ‘deep’ and suggests hidden sadness. One thinks of expressions like ‘well of loneliness’. This shows how their relationship has changed over time.
A Complaint