What if the person feels sorry in heart and does niyat of not repeating but does not feel that he should say in words because of his yakin in mercy?

Imam Ali (as) said: “A pure repentance is feeling of sorrow in the heart, seeking forgiveness with the tongue (by utterance), and the intention not to do the sin ever again.”

So we see it starts in heart then through tongue and then intention

Jabir ibn Abdullah Ansari (ra) says that a man came into the mosque and after offering two units of prayer said, “O’ Lord! I seek your protection and pardon.” Then turning to the man Imam Ali (as) said, “O’ man seeking pardon by such a hasty movement of the tongue is a false repentance.” The man asked as to what repentance means. Imam Ali (as) said, “Repentance consists of six conditions.
1) Repentance of the sin
2) Determination to avoid the sin in the future
3) Fulfilling the duty neglected
4) Discharging the trust if there was any
5) Humiliating or sublimating the self that has revolted against prayer
6) Feeling sorry for the enjoyment realized in the past sins.”409
Also in another narration it is said: It is narrated that someone said: 'Astaghfirullah (I seek God's forgiveness) before Imam 'Ali (as). He said to him. “May thy mother mourn for you! Do you know what is istighfar (seeking forgiveness)? Verily istighfar is a degree of the 'illiyyun (people of high station) and it is a word that means six things. First is remorse over the past. Second, the resolution not to return to it ever. Third, to return to the creatures their rights (usurped in the past) so that you meet God Almighty in such a state of purity that no one has any claim against you. Fourth, that you fulfill every duty that was neglected by you, in order to satisfy your obligation in respect of it. Fifth, that you attend to the flesh of your body that had grown on unlawful nourishment so that it melts away as a result of grief and mourning and the skin adheres to the bones, after which new flesh grows there between. Sixth, that you make your body taste the pain of obedience in the same way as it tasted earlier the pleasure of sinfulness. When you have done these things then say Astaghfirullah!”

Imam Ali (as) said “Seek God’s forgiveness very frequently, because it brings about earnings.