First of all, there is nothing wrong with making all your quilts from patterns. Do you really want to design your own? Or do you feel "inadequate" because you don't? If so, quit it! Your originality comes through with the fabrics you choose and the artistry with which you execute the pattern.
If you really want to design your own, you might want to consider using EQ. I use EQ a lot, and I also use Excel spreadsheets. But I'm comfortable with computers and math, and hate the idea of graph paper where I would have to work hard and erase to make a change. Use whatever works for you, and remember this is QUILTING, not math.