When Heiman was first formed, I was unsure how to feel about it. After Eric treats Heidi badly, and even tries to get her killed, I was adamant about that fact that Eric never cared about her, that he was just using her because she made him feel better about himself, and he wanted to get rid of her because she didn't give him 100% of her attention. However, after a recent rewatching, I've come to realize something. Eric doesn't have any doubts in his relationship until his bathroom talk with Butters, who seemed to still be bitter after his breakup, and pushed those feelings on Eric, causing him to doubt his own relationship with Heidi. In a later conversation, he even comforts Eric for being "duped", once again pushing his own assumptions from his breakup onto Eric(Heidi will leave Eric's heart "covered in poop", that all girls hate boys, etc,), and makes him put his whole relationship into question. Also, since Heidi was so into her analysis of the board, she wasn't able to give the answers that Eric went looking for in order to prove to himself(and Butters) that her feelings for him were sincere. It's a shame too, since(after my rewatching) it's clear that Eric was turning for the better when he had Heidi to help.