Dear Rabbi What happens to somebody who really says to himself, "I will sin then repent, sin then repent," then does exactly so numerous times, however, later on realizes that this was his biggest mistake in life without a doubt, feels true genuine regret, begins to live an honest and upright life, is humble to himself and the people around him or her, changes his wicked ways once and for all, tries to repent with full love for Hashem, tries to do teshuvah in all honesty as effectively as possible with full heart, soul and intention, doesnt want to lose the ability to do teshuvah, feels remorse and never returns to the sin again. Does Hashem have mercy on such a Soul ?

Definitely! Your question is dealt with at length by Rav Kook, R. Tzadok of Lublin and others, and the answer is that if the regret is really sincere, then it's always accepted by God. Being told that in these individual cases, one's teshuva isn't accepted, is just part of the challenge to clarify that his resolve to improve is truly sincere, and he's not just saying so in order to go to heaven, in order "not to go to hell" etc.