It may at this point sound cliche and overused... but it is the truth:

You are NOT useless no matter what you think... it is just the depression that makes you believe those negative things. The depression is responsible for all of your other questions too. It is not reality, it is just the "reality" that depression makes us believe.

And for the first question, just remember that depression is an illness, and the negativity of depression also plays on itself. But there is hope and treatment and relief.

Keep listening to your friends here to remind you of the truth about how useful and wonderful you are, and try not to listen to the nasty depression.