I have macular degeneration and I do not see straight lines. But I have found two things that help me anyway. One is the June Taylor Shape Cut ruler. It guides my rotary blade exactly the way I need it to. I can cut strips, squares, and rectangles with it, and the sides will be straight. The second is a thinner thread, like Shelley suggests. That makes my ironing lie flatter.
If you don't have a June Taylor Shape Cut ruler, it might be a real answer for you. I had to be shown how to use it originally, but it is easy to understand, once you have done it once.
Don't give up. If you enjoy what you are doing, who cares if it is perfect. My quilts aren't, but no one has ever fussed about them.
Oh, I just remembered, I also changed the quarter inch foot I was using on my machine. I started off using one that had that little thing sticking down for the fabric to bump up against. I ended up usually pushing my fabric under that ridge thing. I was surprised that I was able to sew better with a foot that didn't have that thing sticking down. Maybe try out a different quarter inch foot? I had been quilting about a year before I figured that out. I am on my fourth year of quilting, but about my 55th year of sewing...
Dina