What an idiot, shape up or I hope she leaves you and finds someone to ride with who will enjoy it.
Teaching my wife to mountain bike and our short rides to start out are some of my favorite memorys of the last few years.
Seriously you wonder why she isn't improving? On that old pos bike with no lessons or instructions? Older people don't pick up skills as easy as kids they need instruction.
Buy her a new all mountain bike something used but something good that fits spend around 2500+, get her a weekend at whistler with a beginners girls group or your local lift mountain.
Ride for a week or so then get her a one on one coaching lesson. I bet after the weekend and the coaching you will be riding blues easy and she will have a lot more fun.
Now when you go out for a ride with your wife. Find an area that has shuttle access to hard trails but a good green/blue short trail system. Go for a nice long ride with your girlfriend. then get her to shuttle you for a fun run of your own.