It might not be you. It could be your machine. I thought I couldn't sew straight either until I bought a Pfaff 10 years ago. Before I bought it I was using an old Montgomery Wards machine. You had to sort of sew at an angle to do quarter inch seams because the needle didn't move over and the feed dogs were set too wide for quarter inch seams.
Then I sewed on the Pfaff. Wow. What an improvement!