As-Salaam alaikum,
A man asked the Prophet, Sallallahu alaihi Wasallam, "O Messenger of Allah! What is Imaan?" He replied: "When your good deeds please you and your evil deeds grieve you, then you are a believer."

The man, again, asked: "O Messenger of Allah, then what is sin?" The Prophet, Sallallahu alaihi Wasallam, replied: "When something causes a doubt in your heart, leave it out." [Musnad Ahmad, No. 22166].

Thus the expression of joy and sorrow over our good and bad actions respectively are signs of Imaan (faith). So, let us allow our outward expression of feelings be the signal of existence of Imaan within us... and expressing sorrow over sin refers to feeling really guilty and sincerely repenting.