"Do not dwell in the past; do not dream of the future, concentrate the mind on the present moment." Buddha
"The universe is change; our life is what our thoughts make it." Marcus Aurelius Antoninus
"Life's ups and downs provide windows of opportunity to determine your values and goals. Think of using all obstacles as stepping stones to build the life you want." Marsha Sinetar
There are some explanations on what these can mean to you at the following website which is where I found these. Ultimately though, it is your own personal view and outlook on the world that determines what certain ideas, in this case, in the form of what quotes can mean to you.