My problem seems to be simple yet I can't really countinue this project without a solution.
I've been modeling a building. Then once I switched back from wireframe mode to solid mode and my whole mesh became so weird. Everything seems to be okay in, except that the faces look terrible. If I orbit in the viewport, they - the faces - become half-transparent in various shapes and it all seems bugged, like if I duplicated the mesh and it overlaps itself.
What should I do? I didn't apply any modifier to it except a mirror. Thank you for the answer.