Hi Blaire!
I just want you to know that you are not alone in these feelings. As someone with bipolar disorder I get these feelings quite often. I hope that you have spoken to your doctor or someone about the way you are feeling. I worry because of how severe this seems to me by the way you describe how you are feeling.
It is understandable to get depressed or sad at times and I know that my words will not change those feelings for you. I do want you to know that everybody makes mistakes and does bad things but that does not mean that you are a "piece of crap." You are a strong person for waking up each day and making it through the day. I just want you to know that there are people that care for you and know that you are a great person and someone that is important in their lives. I know that when I am feeling down and I just cant stand to be here I read quotes.
I am going to share a quote with you that always makes me chuckle and feel a little better.

"Just because you are trash doesn't mean you can't do great things. It's called a garbage can not garbage cannot."

I am in no way implying you are trash. I just love it because to me it is saying that even something people think negative things about does not mean it is useless.

It's funny.

I hope you are feeling better. I am here for you as someone who has gone through this if you ever feel like you have nobody.