If you use one of your own previous assignments/essays in your work, it is still good practice to cite it, even though it is not a published work.
To reference your own work you will need:
Student surname, Initials. (Year of Publication) Title of essay/assignment. Module code: module title. Institution. Unpublished assignment.
Parry, A. (2017) Assessment of prevalence and risk factors of diabetes and pre-diabetes in Wales. NAD_7_006: Acute and complex care in Adult Nursing. London South Bank University. Unpublished assignment.
In-text citation: (Parry, 2017)