SP 396 - Alice: How should I confess, how should I live so that no dirt remains despite sincere confession?
JESUS: My child, purity of soul, purity of heart is Love, I am the living one and filling your soul. Talk to Me constantly, desire Me and rejoice in Me. And at the same time be with Me wherever I am. Where I suffer, where I die in the world of human souls of My children who have abandoned Me.

One must confess from the lack of love in the heart, in thoughts, in the look at another person.

Purity of heart is the desire to love Me in everything you experience, what you see in all people. If you do something, experience something, think something, let the main intention of your heart be to love Me and to expand My Love in the world. Only in the background should your affairs be: both temporal and eternal.
Everything in your life has value to the extent that it increases love in human souls and in your soul – to the extent that it deepens the bond with Me.
Your main sin is therefore the neglect of efforts in this direction: the neglect of living communion with Me. Listening attentively to My voice and My will. And the neglect of giving your heart and your daily life to Mary - My Mother, who can and desires to make your heart pure in the image of Her heart.