Rachel's major struggle is against herself and her feelings regarding her cheating husband Mar, as well as her self-perceived personal flaws. The conflict type is "Man versus Self." Rachel is neurotic, especially about small things that don't matter, but her discovery that Mark is cheating on her givers her something of substance to obsess and fret about. Rachel's belief that she has caused Mark to cheat through is a major part of her internal battle; she blames herself instead of seeing the reality of the situation, and so is forced to face herself in a deliberate, although possibly unconscious, personal lie.
[Mark] told me that although he was in love with Thelma Rice, they were not having an affair. (Apparently he thought I could handle the fact that he was in love with her but not the fact that he was having sex with her.) "That is a lie," I said to him, "but it's it's true ... if it's true, you might as well be having an affair with her, because it's free." Some time later... he said that he nonetheless expected me to stay with him.
(Ephron, Heartburn, Google Books)
Rachel's struggle with Mark and his infidelity is the other major conflict -- "Man versus Man." She is unable or unwilling to simply leave him, while he seems to think that she should be accepting of his affair because it makes him happy. This makes Rachel question not only Mark's sanity, but her own role in his life; instead of worrying about herself, Rachel wonders if she had forced Mark to cheat. It is only later in the book that Rachel starts to realize that Mark alone is responsible for his actions; she has a duty to herself and her children, but her duty to Mark ended when he made the decision to cheat.