The major thing you need to realize is that your self-worth is not tied to what you do. Even some of the most successful people in the world feel like a failure at times - this is because they are basing their worth on what they do, not on who they are. And that is a trap. As a famous book says, you are "fearfully and wonderfully made", and this is something you need to internalize - that you are a unique human being with experiences that no one else will ever have. You have two loving parents who recognize this fact. I can hear you at this point raising an objection, "But that's just becaus-". Stop that. Tune out of the negative self-talk and replace it with positive self-talk.
At this stage of life, a lot of things are happening that are adding to your stress. Don't give up. You'll find that this seemingly hopeless situation is a case of "this too, shall pass".
Now for something practical. You say that you feel that dumb. I did too, and sometimes still do. Why not put some of those feelings of inadequacy to work? If you're of lesser intelligence, it means you will need to work much harder to reach a average level of performance. With this mindset, I soon surpassed my classmates in college, despite being a very slow learner and a person of below-average intelligence.
I'm now working at a famous company as a robotics software engineer, and loving every moment of it. Hard work pays off, even if you're of lower than average intelligence. Take small steps towards your goals. As one author put it, "Little by little, one travels far."