Landfills are just dumping ground for whatever comes along. They are places where the things of yesterday may or may not break down. Things there rot and breed flies.
A garden converts the waste from the past that is mixed in it into fertilizer, and uses it to produce fruit in the future. It reduces the past to an investment in the future that is no longer recognizable as its former identity.
What are you allowing to enter your mind? What are you meditating on? The past, or the future?
How do you allow the past to influence your thoughts: as raw material for transformation, or as a layer of rubbish that takes up space and stinks?
Set your minds on things that are close to the throne of God. (Colossians 3:2)