Satan is not omniscient. He is clever and I’m sure very observant. So if he sees how a certain person always goes for chocolate when they are discouraged, he gains some insight.
Putting your struggles down on paper does not give him any extra power over you because his power is delegated by God. To use Luther’s phrase, “Satan is God’s dog.” Sometimes the leash is given more slack, but he never can do more than God allows. Anything he might “read” on paper he probably already knows by his observations and God’s permissive will.
As C.S. Lewis said we tend to either give Satan too much power or too little. It’s tough to find the right balance, but remembering key teachings in Scripture is the best strategy. And Satanic strategies are what we are not to be ignorant of (see II Cor. 2:11).