I take this passage to mean the following: Anne is greatly agitated because she is so near the man she loves, who is terribly angry with her, and therefore she simultaneously longs for and fears his company. But she cannot fully appreciate being so close to him because of the barrier between them. She could not command his attention any more than he could bestow it, because of Mrs. Musgrove. The language used "fat sighings" and "nobody had cared for" gives the impression that neither CW nor Anne is happy that CW has to talk to Mrs. Musgrove. Anne's agitation is screened from notice becuse Mrs. Musgrove is so large, but CW should be given credit for truly governing his looks and tone in such an unpleasant arrangement. It's difficult to know whether CW wants to talk to Anne or be on the same sofa as she, but it seems to me that Anne is momentarily vexed with Mrs. Musgrove for literally and figuratively getting between her and her man.