Am I correct in my belief that God decides how long we are to live, before our conception? That is, while we have a say in how we die, neither we, nor the medical profession, have any control over when. In other words, if a person commits suicide, while the choice and method are a sin, they would have otherwise died another way. For your information, I'm not trying to suggest that we shouldn't take proper care of ourselves, or refuse to go to the doctor, as I realize they are the method, as it were, of staying alive, even if not the cause.
Let me pass along a response to a question similar to yours. The response includes the thought of taking care of the bodies God has given us that you expressed.
“A clear passage that speaks of all our days as already known by God is Psalm 139:16. ‘All the days ordained for me were written in your book before one of them came to be.’ Nothing happens in this life without God permitting, guiding, and directing it to serve his ultimate purpose.
“However, this is one of those places where we must realize that Scripture says ‘two’ things that will seem contradictory to human reason. Scripture also clearly indicates that we are responsible to God for actions we take and decisions we make that impact our life and health… Trust in God does not mean that we forego wise care for our health and life.
“So, here are the two things that we must say in order to keep our biblical balance. Are all our days known by God—exactly what they will be and how they will end—before I ever live them? Yes. Are we responsible for our actions and decisions that impact our health and life? Yes.
“And how can we with our human reason reconcile how both of these things can be true? That is where we leave it up to God whose ways and thoughts and reason are far, far beyond ours (Isaiah 55:8-9).”