I have hit my breaking point! Any advice?

I am 18 and I completely had a melt down yesterday. I have had bad anxiety/depression that I have been trying to hide for a few years. I didn't want my parents to have to deal with two mentally ill kids becuase they have a lot of trouble dealing with my sisters bipolar disorder. Anyway my stress and anxiety has been building and building as i get older and feel like I need to be more on my own. I have never really gotten along with my sister because we are polar opposites and she has never been nice to me my anger towards her keeps getting stronger so much that i don't even want to be around her or hear her in the house. I feel so trapped here because my anxiety keeps at home all though I desperately need to get out and have other things to focus on. Anyway yesterday she was singing and playing the piano as usual and I got irritated and yelled at her to shut up, she then started cussing me out at the top of her lungs and I snapped in went after her pulling her hair and hitting her. It scared me because it is so out of character for me and even though I don't like my sister I wouldn't physicaly fight with her. I was panicing the whole rest of the day and ended up revealing my anxiety/depression to my parents. My parents are obviously pissed and I am grounded for the first time in my life (I have never liked getting into trouble). My parents will not let me be in contact with my boyfriend at all for a week which I am having a lot of trouble with because he is the person I am closest with in my life and I don't have other friends. I really feel like I should be talking him or out of the house I know they are trying to punish me for what I did but keeping me cooped up and isolating me is making it worse =(