Did you tighten the cog and lockring with their specific tools, or did you use some other form of tomfoolery to tighten it down?

Also, hope is not lost. You can thread a track cog on the freewheel side of your hub, and tighten it down with a bottom bracket longring ($1 or so at most bike shops). As long as you tighten like the dickens and use a brake, you'll be fine.