When should you "let go" of an SEO Client?

I've been in the SEO industry for about 6 + years now and run my own SEO agency. I started my business about two years ago and have seen a lot of great success with the clients I take on, except for one.
This one account was my second client that signed on when I went independent and from the start the relationship was challenging. The director of marketing over there is really difficult to work with (opinionated, thinks she’s always right, doesn't believe in SEO, etc.). Every meeting with their team could be a good meeting or I get chewed out for an hour. This happens about 50/50.
As I continued to work with her, I took it on as a challenge. Because I wanted to do great work, provide them with great results, and change her perception of SEO and the value that it brings. One problem though is that she only approves maybe 50% of my work to be implemented on the site (and that’s being generous). And the things that I believe will be HUGE game changers she doesnt approve most of the time because she doesnt see the value in it or doesnt want her team to focus on implementing a project that she sees as a waste of time.
Their industry is super competitive and I’ve discussed with her that she needs to be more dynamic to better compete. Over the last year, their organic metrics have seen a continual decline over the previous year. At this point, I feel like I’m running in place and out of strategy because I know she won’t approve it.
Does anyone have any advice? Should I ‘let them go”? If so, what do you believe would be the best approach?