What's wrong with me? (M 20)?

I'm honestly doing pretty good in life right now, i'm getting through college. I workout almost everyday. I'm about to start a job where my pay is way better than right now. As an introvert I've been really pushing myself to be more sociable. Basically, i'm winning on all fronts of my right now but I can't shake these terrible feelings I have... Every day I feel an overwhelming sense of dread, angst when it gets the better of me I have cycles of very intense depression. I feel like something I cannot control can bring down the house of card that is the life I am building for myself. I feel like if I died tomorrow everyone would forget about me eventually. I struggled with these feelings for a long time, I had a rough childhood but I am doing a lot better now. Why do I still feel like this?
What's wrong with me? (M 20)?
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