Cherophobia, the Fear of Happiness.
If you look all over the web to find sources on this, there isn’t really a reliable definition. By reliable, I would mean that cherophobia is cited in a scholarly review paper. The closest to a medical diagnosis is from rightdiagnosis.com here. It defines cherophobia as ‘An exaggerated or irrational fear of gaiety or happiness.’ It too doesn’t provide a lot of details into the condition. However, another source here did talk about it more. Apparently, this condition may have occurred because of past disappointments. To me, this makes sense, because what else could possibly make a person afraid of being happy?