Lorraine,
know how you feel as I'm about to lose my right big toenail for the second time in 12 months (did anyone tell you they take 10 months to grow back?)
The advice about size of shoe is really important. I'm normally a size 5 - 5.5 but my running shoes are 6.5 (New Balance which seem to come up small).
This might be really gruesome but you need to have a look at your toenails as soon as you realise there is damage and see if there is a) blood or b) other fluid swelling under them. The best way to avoid losing them is to relieve the pressure under the nail as soon as you realise there is some building up, then tape them down with sports tape. this prevents the nail lifting away from the nail bed as if this happens near the cuticle the nail 'dies' at the root and falls off. Chiropodists might have a fit reading this but it does work.
Relieving the pressure for me involves a 1mm drill bit and a needle (both sterilised). If you've not thrown up yet and are still interested I can tell you how to do it but it is NOT for the faint-hearted or squeamish. However, guarantees instant pain/swelling relief if you hit the right spot.
Bear in mind that if you damage your nail bed permanently your nails might not grow back at all!