I m very good at physics and chemistry but weak in biology?
Avoid biology and stick with courses in the physical sciences.
what should you do about what?
Probably you're just not that interested in biology, that might be the reason. Because physics and chemistry are a lot harder than biology, to be honest with you. Biology shouldn't be a hard subject to study because it's mostly just about memorizing lots of big terms and taking notes. When I was in high school, I was just like you. Because I don't like biology.