The death of his mother marked the first personal loss from a parasyte. At this point, I don't think he can "get over" it any more than he has. Having killed the parasyte responsible, the only way for him to come to a full resolution to either eliminate all parasyte threats, or accept them as part of society. This is especially the case since Migi now has to sleep, so Shinichi will always have a window where he can only protect people within his immediate vicinity.