Why do I keep thinking about divorced parents?

Yesterday, I saw photographs of my parents when they were happy. They looked so great—so happy. Now, since they are divorced, they changed a lot negatively. This has a big impact on me negatively. Today I cried myself to sleep thinking about how great it would be if my parents got back together and we would be one happy family. Sometimes I just want to punch myself for thinking and “obsessing” over these stupid thoughts. How do I stop thinking these impossible thoughts?