In Mrs. Dalloway, what do Peter's violent protestations that "he was not in love with [Clarissa] any more!" tell us about his character? It seems like a case of he doth protest too much...
The character of Peter Walsh in this wonderful novel acts as the foil for Richard, the man Clarissa eventually married. Peter is a character who is shown to be ambivalent about so much and lacking in self-awareness. This is shown through the way he spends so much time and effort trying to convince himself that he no longer loves Clarissa. However, in spite of these efforts, it is clear to the reader that the grief he expresses about losing her and his obsession with her strongly suggests that he still loves her a great deal. Note for example the following quote:
For why go back like this to the past? he thought. Why make him think of it again? Why make him suffer, when she had tortured him so infernally? Why?
The repeated questioning and the way that he brings up again his past pain at being rejected by Clarissa strongly suggests that he is not "over" her and still feels strongly attached romantically towards her. Note how, even when he tells Clarissa about his new love, he is unable to stop himself from weeping. This again suggests that he still loves Clarissa and that he is a character who is blind to his own feelings and emotions.