I go through the same thing, and everyone deals with it a little different.
What works for me is reminders to myself on the following lines:
1. Your long term gains are largely driven by the price you buy at, as you will be selling when you plan to sell. By being able to buy additional shares with new money at a lower price, you are improving your long term gains.
2. The price the market tells you your shares are worth is entirely unrelated to their underlying value and cash generating potential.
Or, I go back to this old Warren Buffet quote:
“If you plan to eat hamburgers throughout your life and are not a cattle producer, should you wish for higher or lower prices for beef?
Likewise, if you are going to buy a car from time to time but are not an auto manufacturer, should you prefer higher or lower car prices?
These questions, of course, answer themselves. But now for the final exam: If you expect to be a net saver during the next five years, should you hope for a higher or lower stock market during that period?
Many investors get this one wrong. Even though they are going to be net buyers of stocks for many years to come, they are elated when stock prices rise and depressed when they fall. In effect, they rejoice because prices have risen for the “hamburgers” they will soon be buying. This reaction makes no sense. Only those who will be sellers of equities in the near future should be happy at seeing stocks rise. Prospective purchasers should much prefer sinking prices.”
–Warren Buffett, chairman’s letter, Berkshire Hathaway annual report, 1997