Self harming
Self harming is a symptom of some mental and personality disorders. People, for example, cutting themselves because they get a temporary sense of relief in doing this. Jesus in Matthew chapter 5 verses 29,30 and chapter 18 verse 9 talks of plucking out your eye and cutting off your hands and feet which if you physically did this would be an extreme example of self harming. However, Jesus isn't speaking of literally physically doing this but is speaking of what spiritually is a good self harming.
Jesus here is meaning that we need to radically deal with our sin and what leads us into that sin. When a sinner is converted they repent and believe on Christ. Their repentance and faith involve seeking in Christ forgiveness for our sins and help from Christ to turn from our sins and follow Him. Repentance and faith for that matter, are not just for the beginning of the Christian life but are lifelong. We should not think as believers in Christ that simply in the course of time we will stop sinning in the way we are presently doing so. The reality is that the sin in our lives left undealt with will only increase in power. As believers in Christ we cannot afford to adopt half measures in dealing with the sin in our lives or the things or people that usually cause us to sin in that way.
This is what Jesus was making clear in His unusual language about plucking your eye out and cutting off your hands and feet ! All that we know of in our lives that is displeasing to Christ or often causes us to displease Christ needs to be dealt with by us with God's help. We are actually spiritually harming ourselves if we don't.
“Lord Jesus, think on me
And purge away my sin;
From earthborn passions set me free
And make me pure within."