A long time coming

So I am going through the letters I sent to Peter back in the day... Each one is like a mini time capsule. I am reading about events and situations that obviously were very important to me at the time, but now I can't even recall the event. Amazing the power of the passing of time. I have forgotten so much in just 15 years. How does ones brain decide what memories stay burned forever in our brain and which ones fade into oblivion?

Why do I have vivid recall of inconsequential mundane events, like Peter and I going for a drive one random Saturday, but can't recall the event I was so excited to tell him about that I had to write him a letter? What will I remember or forget 15 years from now? How do you live in the present, without losing the past?