Realising the negative effects from sinning is something essential for anyone wanting to repent. It makes you realise what has resulted from sinning and now needs to be rectified. It is like a doctor who can only prescribe the right medicine when he know what is wrong with the patient. Proper teshuva can only work if a person realises what damage they have done that now needs repairing.
In order to know the true nature of something we must look to where it first appears in the Torah. The true negative effects from sinning can therefore be gleaned from the first sin done by Odom HoRishoin. He caused that he was distanced from Hashem, thrown out from the spiritual place of Gan Eden to this physical world meaning that he now also became physical in his body (perhaps similar to the change from a Malach to a human). He went against the command of Hashem and thus rebelled against the King.
Besides these that are directly concerned with what happened at the moment of the sin there are always ramifications that followed in the wake of sin. A sin desensitises a person and makes them more open to sinning again – Aveira Goreres Aveira – be it to repeat the same sin or now be open to more easily do a different sin. The sin needs to be punished.
ALL these result from ONE sin! Imagine some of us who have sinned countless times, how the sin has changed us spiritually and negatively impacted our spiritual lives! It is these thoughts that must accompany our teshuva to make it real, especially during the ten days of repentance when the whole focus is teshuva and it is a special time for the acceptance of repentance.