The best thing to be is HONEST about everything.....but the first thing you need to be is HONEST to yourself about what is your reasons for SI'ing & I don't know, but I don't always know all my reasons for doing something. Now that I am in a DBT group, it's making me more aware of what is going through my thoughts, but before things would float in & out of my mind without really paying attention to how they were really effecting me.
The thing is that if you explain it & talk it over with your mom, the whole aspects of it that you are aware of, you will be definitely putting yourself in a stronger position. There is something empowering about being the one to explain what is happening rather than just answering questions that we are being asked.
Know it's stressful to have to deal with the issues in our life, but in the long run, it's the only way to get the help we need & start to find the strength we really need to have to live our lives is a healthy way.
Hope you will be able to think rational thoughts about the situation & really help your mom understand where you are coming from & help yourself understand yourself better in the process.

Know it's difficult, but know you can be strong enough to get through this & come out on the winning side.