How can one get rid of repeatedly falling into flirting with girls at work after repentance, and what is the means to make the soul steadfast in that?

The first thing to recommend is to supplicate frequently and ask Allah for chastity. Prayer is the best help to piety and being mindful of Allah, for He says: {Indeed, prayer prohibits immorality and wrongdoing.} Along with prayer, one should strive for presence of heart. Among what protects from temptation is being mindful of Allah and feeling His observation, and hastening to marriage. If marriage is not possible, then fasting breaks lust. The temptation of women is great, and the Sharia has warned against it. One should be wary of leniency in dealing with them through seclusion and the like, and avoid the footsteps of Satan. If the work involves unlawful intermingling, it is not permissible to continue in it except out of necessity. Even with necessity, one must avoid everything that leads to temptation and leave the work when the necessity ceases.