Hi Christina and welcome to the forum,
Though most of us encountered our spinal problems as adults, we all still had difficulty adjusting to no life with constant pain. I can't imagine what this must have been like for a teenager, having to miss a year of school and everything else you've been through. I am so sorry.
From reading other posts, it seems the health care system in Canada, especially for spinal patients is somewhat antiquated. I don't know what your finanacial circumstances are but it might be in your best interest to travel to another country for your 'hopefully' final surgery.
I agree with Maria that you should contact Mark,
GPN Artificial Disc Replacement ADR for help. He can arrange for internet consultations with the best doctors who can then help you decide your next course of action.
A double fusion at your age would probably lead to further herniations in the future at adjacent levels which then must bear the burden of the fused discs. This might have been what your doctor referred to as needing more surgery every 5 years or so. An artifical disc at L4/5 would probably slow that process but also may not prevent further degeneration. Then again, it might.
I've often claimed that choosing your right doctor is your biggest decision. Please contact Mark. He can point you in the right direction and help smooth out your decision making process.
You have a lot to learn and we're here to help as much as we can. Please post your questions and concerns as well as your updates often. It helps to know others experience the same problems. I do hope you find your solution.
Dale