I think this is a difficult time for Lizzy. So much has happened to her and her ackwardness around Darcy is at its greatest with regards to the refused marriage proposal, regrets about her own and her family's bad behaviour and his knowledge of lydia's elopement. She feels so much for him but has been so humbled and embarassed and she is very aware of Darcy's ridiculing eye; she constantly fears him judging her. Allowing herself to become ardently in love is beyond her when she has no peace of mind, no confidence. Darcy had his confidence when he declared his feelings - he expected she would have a high respect for him and would accept his proposal. When the turmoil Lizzy is in dies down then she can get swept away by feelings.