It looks like some of the messages got crossed, so I'll answer them in the order the system will let me.
The trick is to remember that there really ARE both sides--where we get hung up is when we only think about the positive or the negative. Too much "positive only" thinking might make us ignore potential problems, or deny that they exist. Dwelling only on the negative turns everything sour..and we forget the good things.
So when you find yourself stuck in one mode more than the other...remember that life is constantly changing and absolutely nothing lasts forever. When things seem dark, ask yourself what good things in your life balance out the problems. It takes a little mental discipline to remember to ask ourselves if things are really as bad as we think. One book to read that will help you train your mind to think this way is The Feeling Good Handbook