Depression may be the problem. Several symptoms that may be indicative of depression include emotional instability, suicidal ideation and a marked increase in your unhappiness.
You described yourself as a generally unhappy person. Some people describe themselves as a pessimist and others as an optimist but it is relatively rare for a teenager to experience such a long period of unhappiness. It may be a sign of chronic depression.
You also described occasions in which you had difficulty with speech. Are these incidents related to nervousness? Did other people notice them? Have you been told that you “digress into irrelevant topics” or is it your characterization of the situation? The answers to those questions would help me to understand what’s wrong. People with depression often have a negatively skewed view of their skills and abilities. Depression increases the likelihood that you are incorrectly characterizing your speech problems.
If you are in fact having speech problems, then you should be evaluated by a physician to rule out a medical cause. Problems with speech are sometimes indicative of neurological problems.
My overall recommendation is to be evaluated by a mental health professional. You are clearly suffering and have been, for at least three years. Your unhappiness is intensifying and you’re often thinking about suicide. Those are all signs that help is needed and is long overdue. Please take care.