I can imagine that this is a very uncomfortable space to be in. But my suggestion is to deal with this sooner rather than later. The relationship as you thought it would be is over, so don’t prolong the agony for you or him. First of all, he deserves more than what you are giving, and his tolerance for the less-than-adequate relationship will be something he will have to educate himself about in therapy. But as far as your side of the equation, I would be direct and let him know that your feelings have systematically been diminished and that you are not fully present in the relationship. I would find the courage to end it and try to come to grips with understanding the dissatisfaction you feel at the end of relationships.
Here is a link to an article I’ve written about how we are attracted to others and what we might be able to do to alter it.
As you make these changes in your life I would strongly recommend that you begin therapy to help understand these feelings you have. The find help tab at the top of the page will help you locate someone in your area who can help.