I'm so sorry you're still in so much pain.
I have severe scoliosis and have had two fusions, 30 years apart. I'm 51. I fused just fine the first time, but they did very little correction (long story), so I had a permanent 75 degree curve. I always had pain, but the surgery at least lessened it. I worked all those years until I got osteoporosis in my early 40's which broke down my fusion and put me into horrible pain. I was quickly heading toward a wheelchair. It took a couple of tries, but I found a wonderful doctor who came up with a plan for me. He did another fusion, going up into the old fusion and strengthening it and going all the rest of the way down. I'm now fused T4-sacrum. I still have pain everyday, but it's more easily controlled with pain meRAB and I'm no longer seeing a wheelchair looming in my future. My doctors will not release me to work anymore and happily filled out all the paperwork for disability for me.
I'm curious what kind of injections you were getting. 20-30 a month is a lot! Won't your doctor give you anything strong for pain? I would suggest getting another opinion, maybe more than one more. Could they give you any reasons for your not fusing? Are you a smoker? Were you taking NSAID pain relievers, like ibuprofen? Both smoking and NSAIRAB have been shown to interfere with the fusion process, but not all doctors pass that important information along to their patients.
The more levels you have done, the higher the risk of nonfusion. My doctor threw everything he could at me. He used BMP (a synthetic bone protein that gets mixed with the donor bone to help stimulate bone growth), gave me a bone growth stimulator (a device about the size of a beeper that's worn externally and senRAB electrical impulses to help stimulate bone growth), and had me wear a good solid plastic brace to keep me from bending at all so I wouldn't accidently mess things up. He also used the 360 approach, which means they go in from both the front and the back. It's a harder recovery, but it has a higher success rate.
You're young, so you should be able to bounce back from this difficult surgery a little easier than us old folks.

The fact that you're still recovering after so long is a clear indication that things aren't right. I hope you can find a surgeon who will re-do the fusion and that everything goes well this time. Don't give up! It'll be so worth it to get a good strong fusion in there!
I wish you the best!
Emily