Losing inspiration because of difficult painting job

I ended up having to agree to painting a series of photographs for a family member in exchange for money I needed. Problem is, some of the photos he wants me to paint (they're of cats we had as a kid) are just way too difficult and beyond my ability.

I'm doing my best, but I'm beginning to lose faith in my painting. Before, I'd love it, and I'd paint for hours and hours at a time. Now I get annoyed and irritated even thinking about it, I procrastinate for days and weeks, and I can't paint for more than an hour or so without getting frustrated and just putting it away for later. I just feel like I'm not good enough, it won't be good enough, and everyone will be disappointed in the end.

Anyone have tips to keep working at it?