Maybe part of your problem is that you are looking at the pieces from different angles. Be sure to cut all pieces from the same angle, same place on your cutting mat, you in the same place your rulers in the same place. In what direction do you try to cut? For me it is easier to cut at a 90 degree angle from me. I always goof if I try cutting parellel to my body.
I hope something someone has said helps you. It is so frustrating to try and try and not get it. If these ideas don't help, ask more questions!