Doesn't sound to me like you lack motivation, and all the work you do with juniors is inspiring.
Concerning this Eleanor; my latest injury which was serious did nothing but motivate me to get back out running. I fractured my ankle in 2 places, and tore ligaments. I spent 5 months not being able to run (I swam and cycled after 3 months), and it drived me utterly mad.
I started running well before my doctors and physio advised me too, just 10 minutes a day at lunch time, and slowly built it up to 30 minutes a day. I'm now putting in a reasonable mileage/week, and considering the lunacy of the Haworth Hobble in March.
I guess what I'm trying to say is that if running is in your blood, it will motivate you. I've felt almost physically sick at times whilst watching friends from the sideline on crutches, I've had my share of injuries (two what I'd call serious), and I always come back to running. Why? Well I'm not very good, I'm not particularly fast, but there is just something about being out there by myself.
My favourite time to go for a run is ~30 minutes or so before dawn breaks. No matter how hard my run feels, not matter how crap the legs feel, that sun breaking over the horizon rarely fails to bring a tear to my eye.
I guess the only "advice" I could ever impart would be to let the idea that you might not be able to run, be your motivation to run. I just have to revisit how depressed I've been during the months of my injury .