I have some sight problems, and have worked with teaching other people who do. One of the biggest helpers for me is buying a 1/4" sewing foot with a groove on it. It makes a very straight line, without having to see it. It kind of does it automatically.
Another idea for cutting fabrics with a ruler: Get some of that lightly colored flourescent tape that you or someone else can put on the measuring line that you will need to cut. Very helpful!