Injury, weight loss and exercise?

A few weeks ago I completed FB30 round 3 and started FB Burn. I've recently had to give up half way through due to stress-induced muscle spasms in my back. The irony is that one of my main reasons for exercising is as a stress relief...

I'm utterly miserable and displeased. I know I should be kind to myself... yada-yada-yada but really, how do you move past not being able to do the one thing you really want to?

Stress has really consumed my life this year. I've gained 9kgs and am determined to shed all of it in the new year but how do you stay motivated when you can't exercise?