Reddit mentions of The Double Helix: A Personal Account of the Discovery of the Structure of DNA

Stop second-guessing your choice of major. Keep your eyes on what you actually want, and remember that the steps along the way will all build there eventually. Check in on your plans when you're picking classes each semester, to make sure you're still on course and still want that ultimate goal. The REU and some lab time will all help.

Try reading some science-related books, not actual science but stuff about scientists themselves or stories about specific scientific discoveries. Like The Immortal Life of Henrietta Lacks, Double Helix, Eighth Day of Creation, The Disappearing Spoon, and Surely You're Joking Mr. Feynman. Your school should have copies of most of them, and they aren't textbook-heavy (though not quite as light as fiction novels).

Don't forget to stay at least a little rounded. Someone on just about every recruitment weekend for grad school will ask about your hobbies. I'm pretty sure they're required to do so :) Or you'll discover you and your interviewer both do ceramics and can chat about that, leaving a stronger impression than if you were yet another person talking about science. It's good to be done with the requirements, but make sure you keep up something outside your major, even if it's just ultimate frisbee.