Quote 4
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? (1.59)
Peter still suffers emotionally over being rejected by Clarissa. He can’t help but constantly return to the past in order to try to make sense of their relationship.
Quote 5
So she left him. And he had a feeling that they were all gathered together in a conspiracy against him – laughing and talking – behind his back. There he stood by Miss Parry's chair as though he had been cut out of wood, he talking about wild flowers. Never, never had he suffered so infernally! (4.20)
When Peter meets Richard, he immediately knows that Clarissa will marry him. He has been replaced, and feels abandoned.
Quote 6
I was more unhappy than I've ever been since, he thought. And as if in truth he were sitting there on the terrace he edged a little towards Clarissa; put his hand out; raised it; let it fall. (2.62)
Peter recalls the misery of being rejected by Clarissa. Even in present day, he feels the misery just as keenly as he felt it before.