No energy leaves our existence. It's always returned in another form - when wood burns it creates heat and light energy etc. When humans die, our bodies eventually turns into base elements, even our bones. We can acelerate the process via cremation etc, but the end result remains the same.
If you can accept that self-aware conciousness is a form of energy, then you must accept that whatever we are will continue to live on in another form.
However, that isn't what you need to worry about now, just remember: Treasure every moment, live for them, because they'll never come again.