Is the questioner considered sinful if he leaves his fiancée who constantly threatens him with divorce and abandonment, despite his love for her and her need for him, and his feeling that the relationship is unstable?

If the divorce occurred before consummation, it is less severe. It is advisable to forgo marrying this woman due to incompatibility and her bad character. There is no injustice or sin in this. Divorce before consummation is considered a minor irrevocable divorce (baynunah sughra), meaning the divorced woman cannot return to her husband except with a new contract, mahr, guardian, and two new witnesses.