Following directly on E's outburst ending with "wretched mistake!" the text is as follows:

Darcy made no answer. He seemed scarcely to hear her, and was walking up and down the room in earnest meditation, his brow contracted, his air gloomy.
These are Elizabeth's observations and what she deduces from them is her power is sinking! But we know that is all wrong.
I look upon this as another hard blow for Darcy. She is unthinkingly telling him that she was afraid of exposing Wickham lest she might betray his (Darcy's) confidence.
From The Letter:
I must now mention a circumstance which...no obligation less than the present should induce me to unfold to any human being. Having said thus much, I feel no doubt of your secrecy. My sister...
No wonder Lizzy hesitated to reveal what she knew. She tells him she wishes she had explained some small part of what Wickham was like - to her own family at least - and he must feel that it was his fault she chuse not to. Her selfexamination, her regrets are likely echoing in his head.