My girl is adopted and now her step mom is abusing that if you were my original child then you will not do this and bla bla bla . So that she is going like she don't want to live more
But also she wants to tell me because she thinks that if I will be know about that I will cry that's why she always cries in her mind or heart but she smiles in front of me. So, my question is how can I resolve it and make her happy forever
Plzz kindly help its so important

You cannot resolve this for her, she must do it herself. All you can do is be a good empathetic listener, and give her love, and support, as she goes through the learning how to cope with life process.

How old are you both?

First... as was mentioned... this is between her stepmom and her. Your really have no part of this. You aren't going to resolve anything. And I agree with the rest talaniman said and will add a few of my own.

Something REALLY important to remember... and it shows your youth. YOU can't make anyone happy forever... if someone isn't happy with themselves they will never be happy with anyone else... not for long anyway. And forever... is a very, very long time. You have to learn to be happy with what you have... because there are no guarantees in life. You might end up with more, you might end up with less, but you better be happy with what you have because that's all you've got, that's all anyone really has.

If she is adopted by the woman she is calling her stepmother, that's not true. She's her mother, or adoptive mother, not 'step.'
Is there a father, and is he adoptive too, or did his adoptive mother marry him later, or what?

You cannot save her. You cannot 'make' her happy. I know this all too well personally.
No matter how much you love her, do NOT become her protector and savior. She could depend on you more and more for everything, and when you get worn out, she then may REALLY be suicidal.
If all your time is spent listening to her troubles, you need to back off for your own sake, and insist that she get therapy.