The sunk-cost fallacy is alive and well in all of us. The only thing worse than wasting the last 8 years is wasting the next 8 as well.
Here is a different fellow struggling with a different relationship problem no less severe than your own. You might learn something from his plight.
The thing I most learned from participating in those threads was how dedicated the OP was to rejecting the one and only solution that would actually solve his problem; namely separating from this woman. The parallel to your current situation (and my own previous situation) is quite obvious to me. It does not yet seem obvious to you.