When one is in the trough of a wave, it is difficult to see anything but the storm around you. You currently in a trough, Fos. It is not permanent. You are also only 19. Most adolescents think they have the world all figured out and it takes another 10 years or so to discover they were wrong. You are too young to be making the assertions are you are making. You may feel these things and your feelings are real, but you don't have a grip on actual reality.
Sometimes a person needs a boost to see over the obstacles, whatever they are. If you truly have a diagnosed mental illness, you need to be treated. Perhaps even inpatient treatment for a time. To Fnord's questions, I would add: Are you getting treatment?
It is patently not true that most people with developmental disabilities are miserable and hate life. I have worked in the field for about 20 years now and I can tell you that most are happy and engaged in their lives.
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The river is the melody
And sky is the refrain - Gordon Lightfoot