Struggling with my appearance and other issues

I am a therapist who just started seeing clients under supervision and i am in grad school. I am 25. But i look much younger because i am short, have a round baby face, and most people perceive me as 16-18. I try to dress more professional and put on makeup at my therapy sessions. But even that doesn't make me seem much older. My last 2 clients were dropouts and it has caused me down a spiral. My supervision group doesn't have any dropouts. I question myself a lot on what am i doing wrong. I have told my supervisor about feelings of inadequaency, and other fears about being a therapist. Last session was going great until my client went to the secretary of the office and demanded another therapist because "i am not an expert" (mind you, in this office we are all grad students). I was very surprised because I thought it went pretty good. Client opened a lot of emotions, talked a lot, reacted positively to my questions etc. I thought I could hold a safe space for them to open up. I really don't know what am I doing wrong. I am reading technique books and I can't find what am I doing wrong. My supervisor didn't mention anything I did wrong. Yet I think my impression on clients is not very great. I have been buying clothes and trying to seem more mature. I really don't know what else to do.