As bright as you are, you are not a mental health professional. You are trying to make a diagnosis without the education or training or experience to do it. I understand that you are worried about yourself. I absolutely agree that you need to figure out what is going on with you. But I think you owe it to yourself to stop seeking answers from an amateur (you).
You are asking important questions about yourself, your potential, and how to live your life. You’ve tried various “treatments” for your sense that something is wrong (alcohol, risky situations, a relationship) to help yourself. I admire the effort but think it’s been misplaced.
Please get yourself to a qualified clinician for an evaluation. Bring your letter and this response with you to give the session a jump-start. Be as honest as you can be. Once you have finished the assessment, the therapist will work with you to develop a treatment plan to help you.
I’m very, very glad you wrote. It was an important first step in acknowledging that you can’t pull yourself out of your funk alone. Please give a therapist a chance to join you in your efforts.
I wish you well.