I have committed many transgressions and some of them even incur the punishment of karet (literally ‘cut off’) causing early death or other losses. What must I do to atone for these and be sure that they were all forgiven?
To the Questioner:
The Tshuvah process includes three parts: Charatah (Regret), Viduy (Verbal Admission), and Kabalah L’habah (Acceptance for the Future). All three steps must be fulfilled in regard to each and every transgression in order to make sure that it is forgiven.
That means: 1) Regret- You need to first understand in each case why what you did was was so terrible. Only then will you feel a real sense of regret for what you did.
2) You then need to pronounce a verbal admission. It is necessary to say this admission while standing, slightly bent with humility and shame. The minimal annunciation which is required according to the Rambam is: “Please G-d, I have sinned unintentionally, I have [even] sinned intentionally, and I have [even] sinned rebelliously before You and I did such-and such; I am regretful and embarrassed from my actions and I will never go back to doing this again.” Ideally, however, one should articulate quietly (between himself and G-d) all the additional details of what was done wrong against G-d’s Will; the more that he is able to say in detail, the more the transgression becomes uprooted from his heart.
3) Then, once a person has broken his desire for the sin, he must accept upon himself that he will never repeat doing such an act again.
The rule in regard to all atonement is: In equal measure to the amount of his heart’s passion and desire he felt when performing sinning, he must now feel in his heart and express that same amount of passion and desire to do G-d’s Will. (This is called in Hebrew: Tshuvat Hamishkal).
A good way to realistically reach this level of passion and desire to fulfill G-d’s Will, is to invest extra time in learning Torah and hashkafah (Torah world-outlook) from books such as Derech Hashem (authored by Rabbi Moshe Chaim Luzzato z”l) which give the reader an understanding of just how much good G-d really wants to give us. Once a person understands that everything he “was looking for” when sinning was really already included in the good that G-d wants to bestow upon him anyway, he will subconsciously begin to think: “Why should I invest my passion in trying to get what ‘I am looking for’ through forbidden means, if I can get the exact same thing by being passionate about doing what G-d asks of me?”
This realization will cause a person to naturally heighten his level of passion and desire to do G-d’s Will, which will then counter-balance in his heart the amount of passion and desire he had when sinning and distancing himself from G-d.
Once an individual completes all three steps of the Tshuvah Repentance process and also works to achieve Tshuvat Hamishkal, ‘measure for measure repentance’, there is no doubt that he can be forgiven even for such sins that incur the punishment of karet.
 It is quoted in Orchot Tzadikim- The Ways of the Righteous, Sha’ar Ha’tshuvah- The Gate of Repentance, and is explained in Sha’arei Tshuvah, Rabbeinu Yonah's Gates of Repentance Gate 4 letter 5.