Why is Proctor hesitant about revealing about what Abigail told him?
Proctor does not want to reveal what he knows because of the way he came about the knowledge of Abigail's lies. He had been having an affair with her, and when he tells her it is over, Abigail promises to ruin his relationship with his wife Elizabeth if he follows through.
When Proctor tells her, "Abby, I never gave you hope to wait for me," she quickly and angrily responds: "I've got something better than hope." Proctor, perhaps for the first time, understands the depth of her desperation and obsession with him. He is shocked by her hatred of Elizabeth; her accusations are completely untrue and dillusional ("She is blackening my name in the village! She is telling lies about me! She is a cold, sniveling woman and you bend to her!")
John knows that if he leaves, she will do to Elizabeth what she has done to so many other innocent women: accuse her of witchcraft and seal her death warrant. Abby's obsession with John knows no bounds. Finally, he gets it.
John Proctor is hesitant to reveal the fact that Abigail has told him she and the other girls were just "sporting" in the woods mainly because this would indicate to his wife Elizabeth that he has had one-on-one contact with Abigail, his former "mistress". He had previously indicated to Elizabeth that he had only met in a group with Abigail, and this lie would arouse Elizabeth's suspicions.