If your paper is coming out black (or generally too dark), you are over exposing. Instead of 15Sec @ f22, try 5 Sec @ f22.
It may also be worth your while hooking up with a fellow darkroom user close to you. If you were in my neck of the woods, you'd be welcome to pop over...
Shelf life of mixed chemicals varies - Dilute developer is generally disposed of after each session, or at most, kept for 24Hrs. Stop can be reused, but it is cheap enough to dump at the end of each day. Fixer can usually be kept for several weeks, but for important work, it is best to mix fresh quantities of all chemicals.