I used to think that sentence was very important because I felt that some words would last a lifetime. Now that I think about it, it makes no difference whether to say it or not. Some things will change.
I used to think that sentence is very important, because I think some words are a lifetime, now think about it, say not say there is no difference, some things will change.
In the past, I thought that sentence was very important, because I felt that some words were life-long. Now, if you think about it, it doesn't matter whether you say it or not. Some things will change.