There are a number of people that struggle with this exact issue. It is not only a matter of self-esteem, it is clearly also a matter of “body esteem”. Most women and quite a few men struggle with issues of body dissatisfaction and insecurity about their attractiveness and desirability. When this takes an extreme form it is called “Body Dysmorphia”.
Body Dysmorphia is characterized by an extreme dislike or unhappiness with your own body and distortions in body image; for example, thinking your hips, belly or other body parts are bigger than they actually are or exaggerating your less attractive qualities in your mind to an irrational degree. Sometimes personal body shame or dissatisfaction can lead to or increase eating disorders, such as anorexia, bulimia or compulsive overeating, which create their own problems. Frequently, these feeling affect sexual interest and feelings of sexual desirability.
There is a wide continuum between normal body concerns and body dysmorphia, but regardless of the degree of personal unhappiness, it can be lessened so that it does not interfere with your intimate relationship with your partner.
There are several resources for improving body esteem, such as books addressing improving body image and body esteem, body esteem groups and workshops. If the primary issue is a discomfort with sex, there are also resources for improving your sexual self-esteem. I highly recommend accessing some of these resources and if they don’t provide enough support, you might look into groups or individual counseling to deal with these issues.
Perhaps it would be helpful to share your concern openly with your partner and see if he or she can help by sharing the ways they find you physically and sexually desirable. It is also helpful for your partner to talk about non-physical traits that relate to your sexual expression, such as your passion, your sensitivity to their needs, or the quality of your touch. Ultimately, though, these are things you also need to believe about yourself. Look into the resources available to help you feel better about your body and what you have to offer as a person and your intimate relationship with your partner can come back to life.