Depression strikes again...

I don't know why everyday I struggle to get out of bed, get dressed and do my daily routine.  I isolate myself and become highly irritable when I have to be around people.  My niece lives with me and has for almost a year.  She is undisciplined and has had no structure. 
She gets under my skin and taps on every last nerve I have.  She is back talks, disrespectful and is so so loud.  Spoiled too...I am partially to blame there,but I have cracked down.  I don't know how to handle her.  I have never been a parent and I surely would have never dared act like that as a child or even as an adult to my parents.
My husband gets after me cause I lose my patience and sometimes blow up but I am getting better at walking away, although she doesn't ever get it and follows me.  I tell her to give me my space and she is right up in my face and I blow. 
I agree to move to a different state so my husband can be happy.  We had no place to live, so we parked our camp trailer on his friends property.  I had only met these people once prior to moving and really don't know them.  I mean she seems really nice, but her husband gives me vibes that he doesn't want us here.
My husband is never home as he is gone on the road.  I will say he has been home a little bit more since we have moved here, but it isn't the same.  He is busy visiting with friends or doing  things around their yard with our stuff. 
I just feel so alone and truthfully invisible.  If my niece left for a few weeks, I swear I would disappear for good.   
I don't need to be found, there is plenty of wildlife around here, I am sure they wouldn't mind a fatty meal.  I truthfully don't want to be on this planet anymore.  I feel like I am being tortured for being such a horrible person.
Then there is the question "why"?  My whole family is gone.  Not that they really cared much for me.  They were not around.  In my older years, when I was old enough to drive, I would go see them, but it was very rare they came to see me.  I think the last time my dad came to see me is when I graduated High School in 1994.  He died in 2009.  Do they hate me that bad, that they don't want me over there?  Or is it that they like watching me suffer?