Help my mums depressed and making me feel bad too?

My mum has never been completely happy. She's been a single parent with me all my life. She was always depressed because the slightest thing made her crabby and irritable or weepy and down. about 11 months ago she met a boyfriend. he's really nice but they always argue. they ruined christmas, mothers day and a recent family party with their arguing. most couples argue but my mum goes over the top and is always depressed for a week after wards. and i have to deal with her crying at night when she thinks i cant hear, and her saying how she wants to commit suicide. i know she would never do that but it still scares me. what makes the whole situation worse is that she's 8 months pregnant and i have to worry about weather she'll be another single parent. i'm only 15 and i feel powerless to help her. also when ever they argue my grades slip because i cant concentrate on anything so they slip and then my mum gets angry with me and thinks that i'm lazy. i cant leave the house when she's upset because she get upset and thinks that i hate her. i've been self harming for 2 years now and i'm trying to get better but seeing her like this isn't helping. theres no one i can talk to. help? x