Why do we always forget what we want to remember, and remember what we want to forget?

The title pretty much speaks for itself.
I'm plagued by memories of the past, and not necessarily good memories either. Sometimes I want to take a mental shower and clean out my mind. I'm not exactly certain how I can do that.

I want to remember the good days more than the bad days. The good experiences more than the bad. But then again, who doesnt? It definitely is true that we learn best from our mistakes. Partially because we never forget them! Nor do we forget the mistakes of others. It's true that what doesn't kill us makes us stronger, as cliche as that is.

I've just been plagued by bad memories recently. Maybe because it's getting close to the anniversary of my sister's death. Who knows? All I know is I pray my head clears up because it's leaving me muddled. I don't like feeling depressed and weighed down. I should go swimming. But it's too hot. I'll just take a bath.