I think at this point, you really should tell her what she should hear instead of what she wants to hear. This step seems dangerous, but you should basically try to be more honest in the relationship.
Set aside the years that you have coddled your girlfriend and said things only to make her happy. It is time that you say things to make you feel satisfied.
If you disagree with her cooking, say so. If you wish you had married her sister, make that statement, too. Even if your girlfriend ends up dumping you, she will remember the courage that it took for you to be more honest.
Being more honest is guaranteed to keep both parties interested in the relationship. Any good marriage counselor will tell you that honesty is the best policy. Every good relationship starts on a solid foundation of honesty, trust and dependability.