I don't know what a forever cushion is. I have a bothersome back issue, and use a Gypsy Sit Upon whenever I sew. Actually I have one at work, one in the car, and one at home.
http://thegypsyquilter.com/thegypsyquilter.htm
I was suffering some pain at a quilting retreat and one of the ladies said "Try this for a little while." It was this weird purple plastic thing filled with air. I was in love with it as the pain was eased instantly. It is very similar to sitting on an exercise ball, only more stable.
I note that Amazon now has them for sale, and I've seen similar aparatus in yoga studios.
hope this helps.
amh
Saskatoon SK Canada