Hi
Sounds to me like you have problems getting your mom's attention and that she doesn't think that what you have to say or what you are feeling is "important".
I think there is most probably two schools in terms of self-injury. I haven't done it my self but I do get urges to do it. To me, it's almost like hurting my self would alleviate the mental/emotional pain. Others again, like Shadowseer says, do it to get the attention of others so they will realise that something IS wrong. Others again would do it for both reasons.
I don't know what to suggest in terms of your relationship with your mom, but I would definately suggest you find someone to talk to like a friend or even better, some form of school guidance councellor. The best, in the end, would be someone like a psychiatrist.
A lot of times, just talking about things, getting them out in the open and letting it all out helps a lot allready.
I know the feeling of being "shrugged off". Eventually, now that I am older, all the things that my parents thought were whining and whinging, has turned out to be real bad problems which could have been overted had they just listened.
If you feel like talking, feel free to post on the forums or even send me a pm.
Keep well and I hope you don't get too frustrated by your mom's lack of attention.