If it’s any consolation, my D got B’s in physics, despite never having taken it in high school. You just have to put academics first. That means: going to all of the lectures/discussions, joining study groups, going to drop-in tutors, office hours. They have a lot of resources available because it is tough for many, many, many people. Doesn’t mean you don’t belong there, doesn’t mean you will fail. It does mean you will have to dig deep and believe in yourself. And yes, you GPA will probably NOT be what you are used to. Welcome to Cal!