Negativity is an unfortunately easy way to kill a spell's working, even if it was otherwise an effective spell: at least, this is generally found to be the case by those who commonly deal in magic.
Those who are casting a spell in desperation are particularly prone to this mindset. Now, admittedly it is not a good idea to turn to magic out of desperation anyway -- magic is put to best and most effective use when it's used with familiarity as a supplement to one's usual routine rather than something turned to with hopes of instant miracles. You might compare it to a person who opens a jar of face cream and uses it once expecting it will instantly and permanently cure all their beauty woes, versus the person who uses the cream with frequency. The frequent user will tend to be happier with the results achieved.
Nevertheless, whether you used magic as a last resort or whether it's a supplement to some particularly worrisome situation, everyone needs a little help from time to time staying positive while awaiting the magic's manifestation. My usual recommendation is to keep yourself busy so that you don't have time to worry: take the time to watch all those movies you've been curious about, start a hobby, read some books, take up video games or that horrid Farmville thing everyone's so obsessed with -- something that keeps your mind engaged and prevents you from fretting about the magic spell, and ultimately ensures that you don't have time to be unhappy and worried.
There is also an old trick used to inspire a more cheerful mood: force yourself to smile. Even if it's artificial at first, the act of smiling will actually tend to cause your body to think it's already happy, and your mood will adjust itself accordingly. It's an excellent example of the cause and effect reversals that we use in magic all the time.