OMG, you've been working for
3 whole years and haven't done anything ground-breaking?! Seriously though, maybe you aren't 'great', whatever that means exactly, and I doubt it's necessary for you to believe that, whether it's true or not.
You might find (books like) Albert Ellis' New Guide to Rational Living useful - I did! It's about observing and recording your recurring thoughts, particularly ones that make you feel bad, and replacing them with more accurate, helpful ones - stopping the self-sabotage. It's amazing how mean we can be to ourselves without noticing. We're trained to be nice to others, without including ourselves in that. I used to do a lot of extremely negative, paralyzing self-talk - saying nasty things to myself I'd never dream of laying on someone else; sounds like you do this too, maybe. This falls under "How to love yourself", something I had to learn to do. Louise Hay has a great 12-point list of things under that heading, stuff like "Treat yourself like you'd treat someone you really love." Then as you get older, you realize you aren't so terrible, and others aren't so great..
I think 'self-belief' comes naturally with untangling that stuff, and otherwise isn't always a good sign, e.g.
"Thoroughly worldly people never understand even the world; they rely altogether on a few cynical maxims which are not true. Once I remember walking with a prosperous publisher, who made a remark which I had often heard before; it is, indeed, almost a motto of the modern world. Yet I had heard it once too often, and I saw suddenly that there was nothing in it. The publisher said of somebody, "That man will get on; he believes in himself." ...I said to him, "Shall I tell you where the men are who believe most in themselves? For I can tell you. I know of men who believe in themselves more colossally than Napoleon or Caesar. I know where flames the fixed star of certainty and success. I can guide you to the thrones of the Super-men. The men who really believe in themselves are all in lunatic asylums." He said mildly that there were a good many men after all who believed in themselves and who were not in lunatic asylums. "Yes, there are," I retorted, "and you of all men ought to know them. That drunken poet from whom you would not take a dreary tragedy, he believed in himself. That elderly minister with an epic from whom you were hiding in a back room, he believed in himself. If you consulted your business experience instead of your ugly individualistic philosophy, you would know that believing in himself is one of the commonest signs of a rotter. Actors who can't act believe in themselves; and debtors who won't pay. It would be much truer to say that a man will certainly fail, because he believes in himself. Complete self-confidence is not merely a sin; complete self-confidence is a weakness. Believing utterly in one's self is a hysterical and superstitious belief.." - G.K. Chesterton, Orthodoxy
http://www.gutenberg.org/cache/epub/130/pg130-images#id00026