I don't have to look at it now. I know. My first month was the same as yours (July 2003).
Around 2005 I was making about five times more per month than now, with a fraction of the impressions... I attribute this to a shift into a more local audience (from 80% US back then to 80% NZ now) and a surge in the number of publishers (same pie, more slices).
Every few months I read reports saying "online advertising growing to a [xx] billion business worldwide" but really someone is eating my pie.
The advertising business has changed enormously since 2003. More publishers, more pages, more low-quality content farms and other MFA sites (necessitating "smart pricing"), and the advent of programmatic or automated media buying.
Today, publishers (and ad networks) are more accountable than they were in 2003. Slapping up a zillion keyword-driven pages, inserting some AdSense code, and waiting for ad revenue to pour in won't cut it in 2016.
My best year with AdSense was 2014; last year was just under it (but I removed some AdSense impressions and added a couple more networks plus some more direct ads, so overall I was slightly up) My forecast (based on the past few months) is that my AdSense will likely continue to drop, which is why I'm mapping out alternative ways to monetize right now.
I'm not upset about it; everything has a shelf life.
avalon37, same here, 2012 to 2014 were my best, now earnings are half that, back to my 2008-2011 average, when I finally started to make decent online income. Competition for traffic is fierce and the pay gets cheaper. And every now and then you have keep up with new technology or deal with major Google Search or Google Adsense updates that never seem to be in the average webmaster's favor. If I didn't love this industry, I would have thrown the towel.
My total AdSense earnings and RPM have gone up every year since 2008. This last year RPM was only up by single digit % increase. 2016 might be the year I finally see a decrease because like others have done I've diversified my ad network offerings so there's more to click on besides an AdSense ad.