In a dualistic world, we are told that the play of light and shadow, good and evil, has roots in the eternal. Creation is set up that way, and we are caught up in the play of opposites. If this explanation, which is essentially religious, has a strong hold upon you, then decisions about right and wrong become easier.
You can consult the religious system that you adhere to and follow its precepts about how to live as a good person. On the other hand, you may be caught between desire and conscience.
You want to do something, yet you feel guilty or ashamed about it. A married person who is tempted to cheat goes through such a struggle. Society says that the desire or temptation is bad and should be resisted, while remaining faithful in marriage is good and should be honoured. If you value the judgement of society and want to be seen as respectable, the choice is clear. Most of everyday life consists in balancing desire and conscience - doing the right thing even when you don't completely feel like it. People who live successfully within the social system have learnt impulse control. My only comment is that choosing to be respectable is itself a desire, so the choice is not between good and bad. Quite often, the choice is between a fleeting impulse and a more mature desire. The condemnation of desire doesn't make someone moral; it just makes them out of touch with desire.
Finally, I would say that with more maturity, a person can evolve to the point where decisions about right and wrong become less judgemental. You find that your inner guide can make such choices without fearing social condemnation. You are no longer so attached to rigid rules and dictates. A doctor who must decide whether to assist a patient to die, in the interest of relieving the pain of a fatal illness, will make that decision based on very personal considerations. There is no fixed answer in advance. Society rejects too much freedom of choice. It is easy for someone to excuse their own bad actions by saying, "what is immoral for others is moral for me". That is self-centred rationalising, not higher evolution. Yet higher evolution exists, and the world's scriptures tell us that in higher consciousness unity prevails over duality. In other words, instead of condemning evil, a person becomes compassionate toward the wrong-doer and practises forgiveness.