If you are having trouble cutting straight lines try the TruCut rulers and rotary cutter.
www.graceframe.com The cutter rolls down a track on the ruler and you get a straight cut every time. I also put their rubbery stick ons on all my rulers so they do not slip. The rulers won't slide on your fabric. You have to lift to move them but I'd rather do that than have my ruler slip. Also could you find someone to be a cutting partner? I have trouble holding rulers steady because my hands are weak. My husband and I cut out my fabric together. Two or three hands on a long ruler are better than one.