I thought: Every door has only one key.
I forgot: there is more than one key. If you lose it, you can get another one. If that doesn't work, you can also change the lock.
I know: there is always a door in your heart, blocking out the world.
I thought: I held the key to the door tightly.
I don't know when I got lost. I had the key but couldn't find your door.
I don't know when you changed the locks and the key was given to another person.