It can certainly be a little difficult to find yourself imagining positively when everything around us is crumbling. Can you live with negativity in those circumstances? Negativity has no upside. It may however have some usefulness. It could be there to remind you that nothing good ever comes of it. Know of its existence and avoid it!
There is however a difference between negative imagery and coming to terms with the consequences of it's use. Positive reinforcement needs it. If no negative thoughts ever occurred, we'd stop being.
It isn't enough to be aware of its existence, you must strive to control it, use it positively. You won't ever get rid of it, so it serves as a reminder of how things were or might have been.
Positive imagination has an immediate effect on your life. Things fall into perspective more easily. You might even benefit from better health, a stronger state of mind. Think of ways to apply this principle. I have a golf instruction book that relies heavily on this very principle. A negative thought is a liability, a positive one is an asset. By adding into your assets column, you effectively reinforce your account compared to someone who is constantly withdrawing from his account.
If you play golf, you know this to be true. You know you're going into the bunker if you imagine the shot in your mind's eye. If you can do this with negative thoughts, why wouldn't you be able to do the contrary?
So, what do you do if a negative thought comes to mind? Replace it with a positive one! It's that simple. To elaborate, think of the contrary. If the bunker (danger) comes to mind, think of the fairway (a safe place). Your chances of landing in it are far greater than landing in a small bunker. If a raincloud obscures your day, do something you had planned for a rainy afternoon, really.
Reinforce the positive and those rainy days will never burden you again.