Chairboy, don't feel bad. Screw-ups are bound to happen if you fly regularly. The best thing is that it happened while on the ground and, above all, you noticed!!
Worst thing that happened to me was skipping one step in my preflight before an aerobatic flight with a friend. The tanker truck guy had just refuelled the plane and, as usual, he shut and locked the cover.
Tried to move it, "looked ok" (sigh), and off we went.
Test maneouvers: full aileron roll, 2 stops. Hop, 1 stop, ok.. "plop!" off the cover goes. Great...
Due to the design of the aircraft (a Mudry CAP-10B) the fuel remains in place. I nevertheless declare urgency and land.
Fortunately for me, the area where we fly aerobatics is not populated, so no risk of cracking anyone's skull with the cover. I have an interview with the club's security commitee and volunteer to pay for the cover, which stands at some $900.
Again, luck said hello and we found a 2nd hand cover from an Ecureil helicopter which happened to be exactly the same. Price: $90.
Hope this relativizes your screw-up and you realize that was not so bad after all

Daniel