Thank you for explaining these difficult feelings. It sounds like you’ve been worrying excessively and the nature of the worry has changed several times running the gamut from diabetes to HIV, schizophrenia, depression, and anxiety, which in turn have contributed to poor eating habits and insomnia. On the other side of the coin you’ve never had a breakdown, suffered from delusions, paranoia, hallucinations, or been catatonic—nor has anyone in your family. In other words, you have no symptoms, and no seemingly genetic predisposition since no family members exhibit these conditions, and your therapist doesn’t think you have schizophrenia.
However, the other side are the thoughts and behaviors themselves. The repetitive nature of the thoughts regardless of the concern, the intensity and rumination, and the healthy behaviors you’ve engaged in also seem very intense, the gym every day and fasting as a form of weight management and health. The repetition of thought and behavior coupled with the fact your brother has a diagnosis and your mom has OCD tendencies make an Obsessive Compulsive Disorder a good diagnosis to rule out with your therapist. What I mean by this is that these symptoms could come from a number of things, but OCD seems to be the easiest one to explain them all.
This is why you want to start there as a way to figure out what is going on as soon as possible. If it turns out it isn’t OCD then you explore what you and your therapist thinks would be worth exploring. An article about OCD is here. An online quiz you can take to see if you match the criteria is here. and ways to distinguish between a generalized anxiety disorder (GAD) and OCD here.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral