I managed a case of spondylolisthesis/spondylolisis L4-L5 after a skiing accident when I was 15 - was tormented with pain, sciatica and muscle spasms for 10 years before I was diagnosed. Couldn't walk for almost a year, failed a year of high school because of it.
A chiropractor fixed me up after I lost control of my legs when I was 25 - was diagnosed properly at that time and there was 11 mm of slippage. He measured it on the x-ray. After 2 weeks of pure torment with this chiro (thought he was going to cripple me permanently), I was walking better than I had ever been and I still visit one every few months for an alignment and i'm pretty much pain free at 38. I hadn't realized how protective i was when walking, was always afraid to take that next step not knowing if it would flare up a case of sciatica.
If you're going to go the chiro way, make sure you get a good one. i've met bad ones, gone through dozens of them, a bad one can do alot more harm than good. My current chiro has spondy as well so he knows what to do with it.
I'm 38 now, and yeah, still have periodic pain but it's managable and I don't need meds for it - but it's more from the back being tired after extended hours of standing, hard to explain. I can still feel the lump in my back when I bend over.
The biggest piece of advise I can give you is to get your back muscles as strong as you can, your muscles will keep your spine in place - it's the only thing that's helped me over all these year. And get a good chiropractor to take care of you. Exercise your back muscles as much as you can tolerate, and do the exercises the doctor gives you. They may be seem silly, but they do work and it's not going to happen overnight.
In my case, being undiagnosed in those 10 years, I was doing alot of manual labor and heavy lifting to earn a living. Yeah, I suffered for it but it's the best thing I could have done to have the pain free life I have now. The doctors told me at the time that I could be in a wheelchair by the time i'm 40 and I can't see that happening at this point in my life. I won't even look at surgery unless i'm facing a wheelchair.
One drug that helped me was mersyndol, it's not prescription but you have to ask the pharmacist for them. Tylenol 3's did nothing for me and morphine/demmoral was too much.
Best wishes, you're not alone.