It seems like there are a couple of reasons for this. It seems like you're looking at this behaviorally, which is understandable because behavior is very tangible to focus on.

It sounds like you're insecure and feel out of control of the relationship. A measure of control you do have is how you think about the situation and to some extent how you feel (by choosing what feelings to engage with and nourish). The flip side of things getting better people don't want to think about (or sometimes choose to focus on) is that that means they can get worse, and you don't know how or when. You don't explicitly say it, but based on your your child metaphor it sounds like you might be worried that if your partner feels like things are getting back on track because of the affair, then that means they might engage in another affair as a way to manage future relationship problems.

Maybe you're not intentionally sabotaging the relationship, and maybe you're not even sabotaging the relationship at all. Maybe these are very real, valid, rational reactions to pain. The less tangible, more complicated part of this is how we feel. Healing inherently reminds us of pain. So you feel happy, you feel like things are getting better, but part of that attachment is the attachment injury.

Maybe you start crying on good nights because you feel safe and comfortable to do so.