Despite the admission of the chotei that "al ki ein Elokei b'kirbi m'tza'uni kol hara'os ha'eilah", that punishment is only because of cheit which drove the Shechina away, Parshas Nitzavim writes that Hashem will remain hidden, "Anochi haster astir panai." Why is the chotei's admission not sufficient tshuvah to remove punishment and hester panim? Ramban answers that the admission is not complete viduy, but is just a "hirhur halev", a thought of tshuvah without a complete committment or verbel confession.
The gemara in Kiddushin (49) writes that if one is mekadesh a woman on the condition that he is a tzadik gamur, the kiddushin is valid "shema hirher tshuvah b'libo", because perhaps at that moment of kiddushin the mekadesh had in his heart a thought of tshuvah which is sufficient to establish a status as tzadik. The implication of the gemara is that hirhurei tshuvah, thoughts of tshuvah, are a fulfillment of the mitzvah of tshuvah!
I am not sure that this poses a problem for the Ramban. Perhaps through hirhurei tshuvah one can attain the status of tzadik in the sense of negating prior sins and removing punishment. However, (and I think this is implied by a careful reading of the Ramban on the parsha), one is still lacking the relationship with Hashem that could have been forged had the time of cheit been spent engaged in Torah and mitzvos. Hester panim is not a punishment in the same sense of the other punishments of the parsha; hester is an absense of presence and and absence of any relationship. For that rift to be repaired requires more than a hirhur b'lev; it demands tshuvah with viduy and a total committment of ahavah.