I think it really could be difficult for your t to have a strong opinion because it's a complicated decision. For example, she's probably not the best one to evaluate your academic qualifications. Ability to write papers and do well enough academically might not be the most important thing for a practicing therapist but it does matter for getting a graduate degree and for expressing that you understand the concepts you learn. Also, there are many kinds of therapists that need different skills. If you became a social worker, you'd need to have an interest and ability to learn about social issues on a larger scale than the individual. Since you didn't express concern about your academic abilities, maybe you have good reason to be confident in your academic abilities. If you get at least okay grades (maybe a B average?) as an undergraduate, I'm sure you could find a program that you can handle academically to become some type of therapist, but your career guidance counselor would probably know a lot more about how to help you figure that out. When I wrote B average there it occurred to me I don't know what country you live in. I'm in the US and so the specifics of my post might be irrelevant if you're in another country, since the types of degrees and grades and licensing for therapists are probably all different if you're in another country.

Anyway, the last reason I thought your therapist might really not know is the reason you said, that you're young and still developing assertiveness skills. I think the fact that you're aware of it at your age and that it is the kind of skill I think people commonly improve on suggests that there's a good chance you'll be able to improve on it so that you could become a therapist. But do you think it's best to go directly to graduate school to become some type of therapist, or would you feel more confident if you got some work experience under your belt first? Maybe some type of social service job that would be relevant to being a social worker or therapist? Maybe do some volunteer counseling on a hotline? I think the training for those hotline jobs and the practice can be confidence building. Anyway, these all seem like complexities that the career counselor could help sort out, and they might not be appropriate for your t, or she might feel that she just doesn't have the knowledge that a career counselor would have about all of the many options.