Life has a way of circling about. Things we experience as children that cause trauma often reappear in our lives as adults. When we were children, the adults that gave us the wisdom that "things will get better" likely knew that because they lived it as a child. Now, we're the adults trying to tell the children things will get better, hoping they hear us.
Personally, I can think back on times when people promised things would get better. I don't even think I was really listening or believing them. It always just sounded like someone trying to comfort me. I consider it kind and empathetic to comfort someone, but I never gave credit to the thought that perhaps they were right. Perhaps they knew because they had experienced similar feelings and just wanted me to know that it would not be forever.
I now comfort my children with similar words. Not empty words either. I have seen both sides of the problems in my own life, and I learned that things will eventually always get better. How quickly or slowly, none of us can ever say. We give advice on what experience has taught us is the quickest way to heal, but we have no way to be certain it will help the person we are telling in their circumstance. We just want to help heal where we see hurt, and all we can verify can work is what has worked for us.
My children likely won't really hear me, just as I could not hear it, but I will keep saying it. "Things will get better."