You have had a very rough course from your prior spine surgeries and the attempted fusion you underwent unfortunately had a low rate of success. Without mechanical stabilization of the vertebra (rods and screws or an immobilization brace), fusion rates are very low (maybe in the 50% range).
You have lower back pain and sciatica. What is the percentage of both together? Do you have more back pain than leg pain (70/30, 60/40) or the reverse?
The disc prolapse at L5-S1 can possibly be surgically treated and your lower back pain can also respond to surgery. You need to find an experienced and thoughtful spine surgeon to perform a complete work up.
Dr. Corenman