How to tell manager I don’t want to do certain project?

I am currently working on project A and in the middle I was put to work on part of project B which is much bigger and I have no idea how to do anything in it. I am already going through alot of personal life things so it's hard to give 100% at work

How can I bring it up to my manager that I would not like to work on project B without looking bad like I am lazy or not able to handle harder projects?