Being happy with the decree of Allah Ta'ala

Q: Please forgive me for asking this question. Time and again this question comes to my mind so I want to find solace in your answer.

Islam is a religion of peace the only way of life to obtain the satisfaction of Allah. Allah sent so many prophets for the same until our last prophet Muhammad (sallallahu alaihi wasallam). If Islam came to our lives to bring peace, why is that so many Sahaabahs who were devoted followers of Islam, died in a brutal way? Umar (radhiyallahu anhu) was killed by a Jew, Usman (radhiyallahu anhu) was killed by so called Muslims, Ali (radhiyallahu anhu), Hasan(radhiyallahu anhu), Hussain(radhiyallahu anhu) all so devoted Sahaabahs died brutally? And if surrendering oneself brings peace than in what form and how it comes? Islam is perfect so it should make our lives peaceful, but I see the anxiety of heaven and hell, the problems of the world remains and the death of so pious Muslims die in struggle?

What is the definition of the peace? Is it spiritual peace inside and not of the physical bodily peace? I keep asking these questions to myself. We are in the right path still why we struggle more than the others who worship animals and idols? What is it that brings peace and what is the exact peace Allah is promising us? 

Aayat for shifaa

Q: I want to know any zikr for my 9 year old daughter, she's falling sick every month since 1.5 year. I am not able to find any proper treatment. It is like wheezing. Please request for dua for her good health and recovery. She lost her father as well last year.