It does not remove you…
Yes we are* obliged * to confess all mortal sins in number and in kind (and circumstances that change the sin…like it was your brother you murdered).
And if we forget one or to say the number…so long as we are sorry (via the normal requirements for confession) and not hiding the sin…it can be *absolved *indirectly along with the other mortal sins…or if one did not have any other mortal sins…the “at least one confessed with contrition etc venial sin” (one of course has contrition for all ones mortal sins…and one would have confessed it etc had they known it…).
But one is obliged to confess that mortal sin that was forgotten (not concealed!) in the next confession (I would do it soon so one does not forget it…but it perhaps could be argued that one ought to do it like within the year if one does not go frequently) when once one has remembered it.
It is not that one then all of a sudden leaves a state of grace…one only does that by a new mortal sin…(which though can happen if one then says…I am not going to confess that mortal sin even though I know I am obliged to…with full knowledge and complete consent…but that is hopefully not going to happen…)
(and one of course go to Holy Communion …one does not have to wait til one confesses the honestly forgotten mortal sin in confession)
Venial sins can be forgiven in many ways…though yes confession of them (at least some) is very recommended…