Contrary to what you might think, in some areas her self-esteem is above your own. You've got to be pretty impressed with yourself to lay there while your husband pleasures you for a half-hour or more with such a limited sense of reciprocity.
I assume you're doing that because you feel that it is the only way you might at least get a little something in return.
I'm guessing that she's content to let you do that, because she's thinking her gift to you is greater.
I think many "why" questions are a waste of time. I wouldn't go there. At least not until some progress has been made. I would go straight for the progress oriented options.
In what context has the "I'm a bad lover" come out?