Simon,
Yours is as valid as anyone's!
Apart from my heart trying to kill me every now and then, I'm in good shape. If I had a physical reason, like yours, it would be very difficult to get on the bike and get out there. This is especially true for you as your injuries were caused on bike.
I remember when I was 15 in 1986, it was only a few days into what were the last school summer holidays. I came off my 10 speed racer at high speed going over the bars and landing head and shoulder first. I snapped my collarbone clean in half and spent the rest of the holidays in miserable pain. After that had slowly healed it took a lot of balls for me to first of all repair the bike and then get back on it. You had some serious injuries and still managed it, balls of granite I say!