I fall into sin and then repent. But my base self overcomes me and I fall into it again. Does this nullify my earlier repentance leaving the previous sins intact and not forgiven?
In the opinion of the majority of scholars it is not a condition for a repentance to be accepted that the penitent does not fall into the same sin again. It is a condition that the sin be given up immediately and that there should be true feelings of regret in the heart. This should be accompanied by the firm intention not to fall into it again. Now, with these conditions met, if he falls into it again, then this will be considered a new sin altogether, requiring a new repentance and with no connection to those of the past.
And Allah knows best.