GUIDANCE    Would vacation be half as much fun if we hadn't been through hell?, we ask. The pain was worth it when we graduate or have the baby. Some people have enjoyed a terrible childhood: Whether it's fun or whether it hurts, it's all perspective; it's all in your head. Some of us can cheer up by wallowing in morbid songs. The difference between adventure and nightmare is perspective too. Things are seldom as bad as we dread they will be. And it feels so good when you quit beating yourself u...