I really loved this response by David in regard to bitterness
Question 8. Do you now see the way to be pure and overcome the bitterness that comes from impurity? Please explain:
Yes, clearly it is a heart issue. It is a "Me" problem.
I can remember, not too long after committing a sexual sin, that I would be so bitter. It was like a release, to get the guilt and impurity off me.
Other people are bitter too, and they may not be indulging in sexual sin. So the source of our bitterness is really from the heart. And it flares up when aggravated.
But the Lord Jesus is not calling us to be rid of bitterness alone, but the root of it - my heart, selfishness, my deep impurity, my desires & passions of the flesh.
Thankfully, we have ridding of impurity & bitterness at the cross. There is healing. There is restoration. There is life in the Spirit. There is an abundance of grace and mercy. But I must come & sit there at the cross. His hands are open wide for me. To receive & take Him in.