He was struggling for the appearance of composure, and would not open his lips till he believed himself to have attained it.

This seems to be Darcy's approach to the whole proposal. At first he is cannot settle enough to make his proposal.

He sat down for a few moments, and then getting up, walked about the room.... After a silence of several minutes, he came towards her in an agitated manner,

After he makes his proposal, one he probably thought was quite flattering, he is feeling quite fine.

He spoke of apprehension and anxiety, but his countenance expressed real security

Of course her rejection takes him by surprise and he loses his composure.
When Elizabeth brings up the subject of Jane and Bingley, Darcy answers with a calm that is really a pretense (and he abhors pretense, too!)

With assumed tranquillity he then replied, "I have no wish of denying that I did everything in my power to separate my friend from your sister, or that I rejoice in my success. Towards him I have been kinder than towards myself."

Of course, Darcy's assumed tranquity barely lasts to the end of his reply. When Elizabeth brings up Wickham, his composure is strained even further

"You take an eager interest in that gentleman's concerns," said Darcy, in a less tranquil tone, and with a heightened colour. ( More color here! Perhaps a touch green this time for jealousy?)

Elizabeth defense of Wickham, finally snaps all of Darcy's composure. Out pores all the feelings he has been trying to keep inside.