Here are some beautiful quotes from R’Kooks sefer Orot Hatushuva (Translated by Dr Alter B.Z Metzger, 1978, YU Press). In [brackets], I put my understanding of what R’Kook is saying.
Chapter V, pg 38
"Repentence is the healthiest expierence of the soul. A healthy soul in a healthy body must inevitably attain the great happiness of repentance, and in the this state the soul will feel the greatest natural pleasure. The casting out of the harmful substances exerts its virtuous and healthful effect upon the body when the body is perfect in its character; and the spiritual voiding of every evil deed and all the evil and corrupt impressions caused thereby, of every evil thought, of every withdrawel from the Godly emanent content in general which is the basis of all evil, the casting out of all coarseness and ugliness must inevitably come – when the organism is healthy both in terms of its spiritual and material aspects."
[R’ Kook is comparing the act of teshuva to the process of excretion by the human body. A normal human body has a desire to get rid of all its waste, (waste being a symbol of a persons sins) to cleanse and purify itself so that it can continue to function. If a body does not desire to get rid of its waste you would say the body is sick and not functioning. So too with teshuva, if a person does not wish to get rid of their sins and allows the sins to corrupt and pollute their soul, you would say that person is "sick" and not healthy. Hence every normal person should have a desire and will to perform teshuva as it is an innately natural process that is fundamental to our existence]
Chapter VII, pg 45
"It is the nature of repentance to give unto man peace and oppressiveness of mind simultaneously. Man is consoled with even the slightest thought of repentance, within one small point of its great light there rests already the lofty and exalted happiness of an entire univserse. At the same time, it constantly places before the eyes of his spirit the obligations of fulfillment, which save him from arrogance and cast upon him a sweet light, giving great and constant value to his lift.The concept of reprentance transforms all inquities and their confusion, their spiritual suffering and their ugliness, into conceptions of happiness and contentment because, by measns of the inquities, there emanates unto man the profound knowledge of hatred for evil, and love of virtue grows stronger within him with noble strength. Beyond all reckoning and knowledge, he derives from the joy of solace the Divine pleasure which is solely for those who have repented. Most pleasurable of all is this feeling when joined with the refining sense of a broken heart and contrite soul, a soul united with deep faith in redemptions and enternal salvation"
[R’ Kook is saying that the nature of teshuva is paradoxical. On the one hand it gives on a sense of relief and peace of mind (because you have escaped punishment), yet on the the hand it leaves one feeling "oppressed". I would imagine this feeling comes from the fact that although you have repented and asked forgiveness for ones sins and will therefore not be punished, there is still a void that you feel is left within you – You are troubled how you could have steeped to such a low, and even if the sin is forgiven, you yourself feel that it is not forgotten (expecially by yourself or by other people involved, never mind in the eyes of G-d). However R’Kook is saying that it is that precise feeling that makes your teshuva genuine. These feelings will cause you to forever feel humbled, constantly watching yourself. You will be keept from arrogance for you know that you are not a saint and have sinned, but yet these feelings allow you to carry on growing because you are constantly reminding youself and trying to rectify your personality so that these deeds will not occur in the future.]