I want to kms because of my niece
Why does your niece make you want to end your life?
She's treated like the most important person in my family
Does that mean you feel unwanted?  I'm sorry this is happening to you, but you shouldn't kill yourself over it!  You should talk to a therapist about these feelings of yours.
Therapy is for losers, I'll handle it myself
This is not a situation you should handle yourself. If you are thinking about committing suicide, you should get help immediately, there is no reason to wait.