Help: cutting fabric

Ok, I’ve gotten pretty good as sewing in the last months, but one thing I cannot seem to master is cutting fabric. It always comes out just a little crooked, the corners of my squares and rectangles are never in a perfect 90 degree angle, which gets annoying when I sew multiple layers and they don’t match up. I have a cutting mat, a quilting ruler and a rotary cutter. I don’t know what the problem is, maybe my ruler slips, or maybe my cutting mat is too small. Any tips?