Hi,
I can tell you how I nearly dumped one of my sites that currently earns me good AdSense revenue.
2 years ago I signed up with Ken Evoy's Site Build It (SBI) and started creating a web site related to music lyrics. Ken advises to build a site about your hobby, what you love to do, etc.
Well, soon I realized that I was not targeting a niche but my site covered nearly all music styles and artists... I created maybe 20 or 30 pages and started with something else.
After few months I decided that this site won't work and I want to start another site with SBI. Luckily, I saved all music lyrics files to my computer and transfered the site to my own web host, where I host all my other sites. And I forgot about the web site...
About three months ago I checked traffic statistics of all of my web sites. I noticed that my music lyrics site was often searched by all major search engines. I had 468x60 AdSense banners on the site and I was making ~$3 per day.
So, I decided to change the AdSense format and colors and do a little testing and tracking...
I am so glad I noticed that my site was receiving free traffic from search engines and I decided to test different AdSense banners...
Soon I was making $5, $10, $20 per day. I started adding more content and my revenue still increases!
As you can see it is very important to finish what you have started before you can say that it's impossible to make money online... I am very motivated to work on the Internet now and grow my business!
Good luck!