I think that what has been decided will cause a major life change is the most difficult decision because it is something that can tell whether my life in the future will succeed or fail.
I think the decision will cause a big change in your life is the most difficult because it is something that can tell the future, my life will succeed or fail.
I think that what decided to make a change in your life a big decision is what most difficult, because it is what can be said in the future, my life will succeed or fail.