YOU ARE NOT WORTHLESS OR DISGUSTING. Whatever you may be thinking of doing - STOP!
You are only 20, and in no way must you have your life all planned out. You are still trying to figure stuff out so don't worry too much about it. You are not in competition with anyone else so go at your own pace. Trust me, there is no rush! Don't give up on school. It is the only way to get out of your stupid job. If you are failing, then maybe it is time to rethink things and try to work something out. You sound very intelligent so I suspect you are having trouble more because you have just decided to give up, and not because you can't do it. It is so easy to find the ugliness in people. As human beings, we are very critical of each other. When you look for the bad in people, you will find it - A. Lincoln ... Take a second look. Look for the good this time. There is a lot of good in this world. If you don't believe me, just volunteer at one of the many nonprofit organizations that are out there. When you start to help others, you will see the good in this world and will start to feel less bad about your own situation. Have you been in therapy? You have been depressed for so long and that is going to leave an emotional mark. You say you don't have close friends ANYMORE? Care to share what happened?... Perhaps, they are still friends you have just effectively isolated yourself. I have no doubts that you can make new friends.
I understand not wanting to worry your older brother, but maybe all you need is to spend some time with him. Maybe you can spend a few days with him during the holiday break. Does he have kids? I am sure your brother would appreciate the extra help, and you can at least be out of the house for a while and not have to deal with the tension between you and your sister. I am so sorry that she has been a bad sister, but at some point you are going to have to let that go. You hating her does not help the situation. I am not saying you must make an effort to have a relationship, just that for YOUR own good don't hate her. Hatred eats at you. She did what she did because she has her own self-hate issues. If she doesn't want a relationship with you than that is her problem. We may not like our family at times or how certain family members treat us, but you can't dwell on it. You are bringing up the past, and for what? It is not helping you in any way. You are letting her get a rise out of you, and in the end I am sure neither one of you feels great about it. If you have to see her as a stranger, then so be it. But let it go. Respect the fact that she is your sister, and for the sake of your mother don't fight with her any more. You can be civil with her, but you need not say more than hi and bye. If she cannot act like the mature older sister, then you be the adult. The next time she tries to fight, walk away. Ignore her. I am sure she will be so shocked that your indifference will anger her more than if you had had a yelling match.
I am sure your mom does not hate you, emotional stress or no. FYI - All children are the cause of emotional stress to their parents. Caring and loving parents like your mom worry about their kids - you can't turn that off (I have been trying to get my mom to stop worrying for years). Remember too that they are HUMAN. There stress is caused most of the time not by the child but by the simple fact that they do not know what they are doing. I am sure your mom has no idea what to do when you and your sister fight. Help her out a little, and at least you can be the bigger person.
Hey crying and being tired are just part of life. They are a reminder that you are still alive. It is through suffering that we gain compassion and through our struggles that we gain experience. This is likely not what you want to hear, but I am just trying to put things into perspective. Sure things are bad now, but they will not always be so. Focus on the good things in your life - on the things you are grateful for like your mom and your brother. If there is something in your life that you do not like then change it. You have the power to do so. I think you have a while before you need to start thinking about kids, but simply because your father made the mistake of leaving you does not mean that you will make the same mistake. The mistake of your parents are your PARENTS not yours. Instead, learn from them.
I am absolutely positive that when all is said and done, your mom is thrilled to have you in her life. She has sacrificed so much and I am sure as a single mother things have not been easy. Don't waste her sacrifice by taking the easy way out. Don't give those who would love to see you defeated the satisfaction. And please stop looking at things in terms of guilt. Sometimes, we just feel bad, and sometimes bad things happen. Blame, guilt, etc. these are just labels and it will make no difference on how you feel. Do feel less pain simply because you feel guilty? You have to trust in yourself that you can handle and deal with anything life throws at you. I strongly urge you to get into therapy, and I strongly urge you to get checked by your doctor. You could have some physical condition that is causing the depression and medication will definitely help. Of course, you can always come here and talk things out with me. Suicide is not the answer. It will only lead to more heartache, and more problems. You are not ugly and you are not a bother. I hope I hear back from you soon.