Dennis,
Same here, mate. I thought leaving the army and being a window cleaner would be an easy transition to make.
It nearly killed me. I made nearly £50.00 on my first day (two years ago), and I remember thinking how the hell does anyone make a living out of this? I felt like I'd been kicked to death.
Just over two years later; and having stopped smoking; I find I can work a hard six to eight hour day and be nicely tired. Rather than bloody knackered.
Try finding a local window cleaner to show you the ropes. Just phone around and offer them a 'free worker' for the day. Watch him carefully and ask lots of questions.
Also stick at it. Nothing good comes easy, and those with a lesser character pack it in after three months. It's fine to be 'down', but don't give up!
Just out of interest, what Regiment or Corps were you in?