Being sick just sucks. It makes it difficult to concentrate and you do not get the sleep that you need to get better. You feel lethargic which makes you feel like not doing much anything. Sometimes, you feel like you are having an outer body experience. Then you have the medicine which can make feel like blah.
I can only imagine that sin is like a sickness. You do things that go against God which makes it difficult to get the rest you need. It is hard to concentrate on God. God cannot be in the presence of sin so He seems so far away. When you sin, you feel like blah because you know it is wrong.
Simple solution, do not sin. Yeah right! If it were that easy, then all who believe in God and Christ would be sin free, but the reality is that all of us sin. The real goal is confess your sins and repent which simply means to turn away from.
As always, my suggestion is to start with prayer and ask God to show you the areas where you are sinning in your life. Just be prepared because you will be amazed at what He will show you.