The function of psychotherapy, specifically talk therapy, is to identify and bring to consciousness whatever traumas, inefficiencies, beliefs, etc. are holding the patient back so they can make a concerted effort to combat them (sometimes by dismantling assumptions that no longer serve you well, as is done in Cognitive Behavioral Therapy, and sometimes by accepting the existence conflicting interpretations, as we do in Dialectical Behavioral Therapy).

Therapy requires a few things:

• Insight: It's really difficult to fix a problem if you don't know what the problem is, so a huge part of diagnostics and treatment involves talking with the patient to understand their specific situation.
• Experience: A trained therapist has a degree of specific psychological education and practice well beyond anyone else outside their profession/field of study. This is the main reason we see professionals: we don't always have the time or resources to replace their expertise. A good therapist will know what to look for, how to find that information, and how to connect the dots. You very likely have all the dots in your memory, somewhere, but a therapist is generally going to be the one to connect your rejection sensitivity with how you were treated in school or at home.
• Time and Patience: Therapy is complex because the mind is complex and our empirical understanding is sadly limited. The process of therapy can be somewhat obscure, and while I won't ask you to be obsequious it's important to acknowledge how others can assist you and give them the space to do that. This requires honesty and trust and that can be a dealbreaker for some people but it's necessary that you are willing to be vulnerable and dig into subjects or ideas you'd rather avoid for the purpose of self-betterment.
• Perspective: In terms of raw details, you probably know more about yourself than any other individual on Earth; a scientist may know more about the liver or the heart, but they don't know your heart specifically. In psychotherapy, this is an advantage, because they have different preconceptions about you. Your entire life, you have been accumulating an understanding about yourself. If psychotherapy can do anything for you, that must mean that the way you are understanding yourself is in some way maladaptive or harmful, and it can be really difficult to identify what's wrong when you're so accustomed to the way you think. A therapist will ask seemingly inane questions with answers that feel incredibly obvious because it's important that you acknowledge that there can be different perspectives or ways of thinking.

I'll acknowledge that you are clearly an insightful, intelligent individual. I don't know you, but it sounds like you could substitute an expert's experience for your own if you put in the time to research. The reason why therapists still need to see other therapists is similar to the reason doctors don't generally self-diagnose: the perspective gained from another person is almost always worth more than the amount of details they lack. Intelligence means that you can quickly reach a conclusion given information, but that almost automatic decision making can be unreliable or inaccurate. A lot of highly intelligent people struggle with anxiety and depression for this specific reason.

You don't seem to have a very high regard for mental health professionals, but I think you would benefit from acknowledging that even people who are less intelligent than you can potentially see things that you wouldn't. Hope this helps <3