If my kidney transplant was sucessfull can it still reject after 12 years im so scared?

Yes always a concern. After transplantation is always the main concern , as the same disease attacks the kidney besides rejection . you will be happy to know reported cases of 34 years of survival of the graft ( kidney ) . Take good cate of yourself visit your doctor regularly and follow the the advice of your doctor. Better methods and medications are in place , hope for the best.