IMO Charlotte could not show disappointment in her letters for a number of reasons :
1) She had entered the marriage state with Mr Collins fully knowing that it would not be a romantic union, so there was no need of complaining of things she knew would be. In that sense there is some pride in her attitude.
2) I do not think a well bred young lady would weep over her lot in letters: would it not be quite improper (above all when you are no loner a teenager)?
3) Mr Collins might read over her shoulders or find her letters before she had finished writing, so she had to be discreet
4) She knew Lizzie was sensible enough to read between the lines and knew she would learn more when alone with Charlotte.