Seth Godin wrote a great book on this exact question. How to know when you should push through the dip and when you should decide to quit. It's an insightful read:

http://www.amazon.com/Dip-Little-Book-Teaches-Stick/dp/15918...

Sorry if this has been mentioned, but I haven't had the chance or time to read all the comments but...It sounds like you are quitting when you hit The Dip

http://www.amazon.com/Dip-Little-Book-Teaches-Stick/dp/15918...