Low self-esteem is about self worth, whereas people with depression tend to think badly about all things, not just self-worth.
Treating problems of degraded self-esteem needs to be worked along with the approach to the depression, it's not okay to treat them all together.
With self-esteem issues, it's very important to remember that everyone has failures, bad days, depression of some level, everyone looks at themselves in the mirror and goes yuk.
Understand that self-esteem and depression make you feel terrible, but you are not terrible, you are not your illness and you are not the person you currently see yourself as. In fact, you would probably be surprised at how highly people think of you.