I have always had very fleeting career interests. When did you experience that aha! if you were once this way?

I am a PhD student in sociology, but am also very interested in writing (particularly screenwriting, but also fantasy), psychology, and a number of other things. Lately I have been thinking about taking undergraduate courses that would qualify me to apply for medical school (biology, chemistry, etc.) and exploring psychiatry. Is there anyone older who felt like this in their teens and twenties (I should mention I am 26, changed my major 3 times in undergrad, and worked as a social worker before deciding to go back to school for my doctorate) that finally settled into a career they love and can offer me some advice? Thanks!!!!!!