Ok, some big learning curves for you coming up: sex for women isn't just about sex. It starts many hours/days before that!
A woman is a gorgeous complex creature and we have to have many many needs met before we feel sexually excited
At mid life, she will be dealing with many body changes that really screw up her confidence and self worth.
Rather than just thinking about the end goal (the great sex), you need to dial it back about ten miles. Look at her life, at your relationship; is she happy, fulfilled? Or is she tired, or bored, or has low self esteem? Who does all the housework and emotional labour in your house? Do you make time for each other?
Go out for dinner (get her to choose where) Ask her questions about her life, big open questions like 'how are you feeling at the moment?' 'what could i do to be a better husband for you?' 'Where do you see our relationship in 5/10/20 years) Get deep. Talk, a lot. Talking is sexy for women. Learn about her, try to unpick what's missing in her life, what you can do...
If this all sounds like too much effort, then you're wasting her time and you may as well split up. Everyone deserves great sex, and the best way to get to it is to connect emotionally first. And that takes time
Google Tracy Cox. She gives great advice on all aspects of a great sex life
Good luck