You're not depressed because you can't help people, if that's really true and not just hyperbole, but you aren't helping people because you're depressed, assuming you really want to help people. What you're describing is depression. What are you doing to address it?
I'm really sorry you are feeling this way. I don't think anyone can judge why you feel you are depressed as this is your own thoughts about your own life. Clinical depression though is often more than feeling worthless. It has symptoms like not wanting to get up in the morning, always feeling hopeless, disturbance in sleep, eating, sadness, etc. (those are just some). And it is also present for at least two weeks ever day, all the time.
We could give you lots of ways to feel helpful to the world or find value in your life but that's hard not know more about you. But know this . .. just by posting here, you've helped others. Someone else may feel the same way and see this . . . and it sparks them to not feel alone and to know that people care and want to help them (us and you). So, stay in touch with us and let us help. hugs
You sound like you are depressed. I have depression and it does help to volunteer in your community. I volunteer as often as I possibly can and it helps to get out of my head and accomplish something and helping others does make you feel better about yourself. Try to find an organization in your community through the internet that attracts your interest and apply to volunteer. I think it might help you as well as seeing a doctor about possibly being depressed.