[Lumos.]

Hi there. I see you are new. Welcome to TeenHelp.
It's good that you told your mom, but if they won't go away, maybe tell her again? She should take it seriously, if this does not go away. Never give up though, try to get through it. You aren't alone in this. Do you know any things that could trigger the sadness?
If you feel urged to cut try taking a look at this page: Alternatives to Self Harm
Cutting is a very addicting habit. If you've only done it once then try to stop before it gets any worse. It may seem like 'oh i'l just do it once' but that once turns into a lot more. Have you tried to tell your mom about cutting or wanting to kill yourself? It may seem really scary, but once you do it a parent can be a huge help to you.
If you are having thoughts of suicide, then that should not be taken lightly. It's your life, and that is so precious. There is so much to live for, even the littlest things.
When you feel like you are going to commit suicide, you should call a hotline. They are very nice people who want to help you.
If you ever need to talk feel free to PM or VM me.
Hope this helped!