Hi Fluffybetty,
Make sure your NVQ assessor is aware of your physical limitations, which surely they must have to take into account. Dont give up, not at this late stage, even if you decide not to work with horses,. Talk to your assessor, they may surprise you with the support they could offer, and if they dont, at least you know....
I believe it's possible for you to ask to change the yard you train at, although I think possibly you are more tied in because of your horse being there. Could you move her somewhere cheaper with more turnout? Maybe separate from where you are training, so you are not so tied in if you do move to a different yard?