A person who, through neglect, develops a malady in one part of his body, need not, for the sake of consistency, neglect the other parts as well.
So it is with the mitzvot.
The question of “all or nothing” is also invalid from a human, personal point of view.
Though the ba’al teshuvah may wish to see himself as one reborn and to begin his spiritual life with a sense of wholeness, it is important for him to recognize that even in spiritual rebirth it is not possible to take on everything at once.
The people of Israel, in accepting the Torah, did not receive it all at one time.
Rather, the process was a protracted one, from the early preparatory stage of the seven Noahide laws to the acceptance of additional mitzvot in Egypt, at Marah, and at Sinai, to the full revelation there that followed.
Similarly, a child raised to be an observant Jew takes upon itself the full yoke of the mitzvot only after long preparation; years of training and the gradual, step-by-step assumption of responsibility according to its intellectual readiness and practical capacity.
The essential point is that living beings do not undergo sudden, complete transformations.
The caterpillar does not become a butterfly in a single act but as a result of a gradual process, governed by certain laws.
Within this process there appears to be a series of jumps between distinct stages, and these the ba’al teshuvah also must make from time to time.
But these transitions, too, are neither as quick nor as sharp as they appear.
–Rabbi Adin Steinsaltz