Did you ever try to encourage someone who then just ignored you?
That's what happens when Moshe tries to talk to the enslaved Children of Israel. He has a message from G-d Himself that He will take them out of Egypt into the Promised Land! But "they did not heed Moses, because of shortness of breath and hard work." (Shemot 6:9)
Rebbe Nachman teaches that "shortness of breath" refers to a lack of emunah, or faith. How can they listen to Moshe when they have no hope on the horizon?
The next time you're feeling down, notice your breathing. It is deep or shallow? Take a few full breaths. Do things look a little brighter?