A visit with a good therapist may help you better understand the nature of your problems. They can even reveal symptoms you never noticed before. However, there is a surprising amount of variability between therapists, so if their diagnosis does not make sense, it might be a good idea to get a second opinion. Keep in mind though that a lot of therapy will end up being similar regardless of the specific diagnosis - assuming, that is, that you do not have problems with psychosis, mania, or borderline personality disorder. You should also be aware of the fact that comorbidity is the rule, not the exception, in mental disorders. Depression and anxiety are the classic examples. Many people are diagnosed with both and/or they may switch back and forth between them over time.
What I mean to say by all this is that a diagnosis can be worthwhile, but it may not define your mental illness with the clarity and precision that one would like. I would also like to add that, assuming your diagnosis isn't wildly off the mark, rapport with your therapist matters much more than your specific diagnosis and even the particular methods they use to treat you (which, like I said, end up being remarkably similar across disorders anyway). For what it's worth, I do not even know my formal diagnosis. From what my therapists have talked to me about and the problems I have brought up with them, my guess is major depressive disorder (MDD), general anxiety disorder (GAD), and alcohol use disorder (AUD).
Overall, even though I have given you a mixed bag of positives and negatives, I would recommend seeing at least one therapist before you throw in the towel. As you are a minor though, I'm aware that this will not be as private and under your control as it should be ideally. You know your parents better than I do, and their likely response to a diagnosis is something to consider.
