I use to self harm when things just go too much but I haven't needed to for a while. I know people are always saying, oh it is just a cry for help, but if you notice people who self harm, rarely tell anyone, if they wanted attention then surely they would tell someone what they have been doing. It is a release, and if you feel you have problems that you cant talk to anyone about, or feel embarrassed, you just need to release the frustration.
I am sorry to hear you are having problems, and both daughters need support, depression is an illness and not an excuse, but also be careful that your youngest daughter, doesnt use depression to explain everything. I forced myself to get over the depression, but I understand that all depressions are different. But you have to want to feel better.