He who understands the gospel will never fall from the state of grace – no matter what sins he commits – because to understand the gospel is to experience perfect contrition, and perfect contrition is the abolition of Hell. If you are unable to detect perfect contrition within yourself, you have not yet understood the gospel. Someone who understands the gospel with their mind, naturally overflows with both Joy and Sorrow in their heart: Joy at the unconditional promise of an eternity spent with God, sorrow at the reality that he daily fails to live up to his destiny.
It is impossible to force your will towards love for God, and so it is impossible to manufacture perfect contrition. You must first recognise that God loves you and all those who you care for, and therefore could never abandon you to damnation. Only then will real love for God spontaneously explode in your heart, driving out any fear of Hell.
But even after all this, there is a higher way. Christ was willing to suffer damnation for the sake of the salvation of his enemies. Your love for God and neighbour is not perfect until you find yourself in the position where you are truly willing to do the same. Only when God has promised you eternal bliss, and yet you are willing to sacrifice it for the sake of the salvation of the other, can you truly be said to have achieved union with the divine.