Mental health problems are real, painful and sometimes severe. If you are doing reckless things that could hurt you, then you really need treatment from a professional, which might include some type of rehab treatment.
Psychiatric hospitalization may or may not be recommended...but to get help, you need to let go of control and follow directions.
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Psychiatric evaluations so that you can know what to possible expect.
Do you exhibit other behavior or emotional problems? Is the self cutting/injecting the only thing that you keep to yourself?
In order to understand and start taking actions to prevent self-injury, you can start by becoming aware of which situations are likely to trigger your urge to hurt yourself. Then, make a plan for what you will do instead of cutting or injecting when you feel this urge.
Mental health problems can be treated. To find help, talk to your parents, school counselor or health care provider.