How do you not get depressed and intimidated by the intelligence of others?

School is about to start up again for me. I'm a junior level major in pure mathematics. Took analysis and algebra, but now that I get to higher material I feel like a fucking retard sometimes. Reading a book on number theory that I said I would do over the summer (Apostol, also some commutative algebra stuff) and exercises that seem so trivial are taking me forever. Less than 2 weeks until classes start now, so there's no way I will learn that much. I just feel woefully inadequate and that's why I couldn't force myself to read this book; every time I struggle on an exercise that should be simple I literally want to kill myself. It seems like all my classmates are brilliant people that are just naturally bright and gifted (although I don't know how much they study and I've been fairly lazy).

It's just so hard to motivate myself now when I seem so fucking stupid, now. Not very conscientious, but would it even make a difference? It's like I'm just too stupid to do as well as others and I pretty much see nothing but futility in trying to do anything but codemonkey after I finish this degree.