The illustration of suffering making the Christian better is in John 15:1-8. The Christians are the fruitful branches while the false converts are the branches that are cast into the fire. Because God sees that the Christian will inevitably bear fruit then He wisely prunes away what causes the fruit bearing to be inconsistent. The parable of the sower talked about how good soil hearers bear different degrees of fruit as a result of salvation. To make a bountiful harvest one must diligently work to get rid of the hindrances. Christians need suffering in order to bear much fruit. When it comes to God pruning the branches the lesson has always been to make Christians do better and not to be bitter towards suffering.
I do not dare deny Christians can be bitter at times like they can also fall into periods of carnality but they do not become bitter indefinitely. As the Christian growth has not always been moving up then there are periods where they can become bitter towards suffering. It is sometimes possible to question God at times and ask why He allowed Christians to suffer while the wicked to prosper like Habakkuk did. God has His purpose in allowing Christians to suffer like by taking away their material blessings so they will learn to rely more on His provision. Becoming a Christian was never a guarantee you will win every contest, become popular and on the spotlight because as Jesus warned many times, if you are going to follow Him then it may even cost you your life. Fortunately, God gives grace to the Christian to endure to the end.