Playing guitar

I have taken up learning guitar [in September], but am finding it very hard to get my left hand to move fast enough!! I had a left radical mastectomy and reconstructive surgery 7 years ago. I have no feeling in my left armpit, but no obvious nerve damage to my fingers.

Am I clutching at straws or is it possible that my fingers just don’t react as quickly as they used to, hence my very slow progress? I need some honest feedback if anyone’s had a similar thing?!