Beachy Head is a landmark important to mariner's returning home after a long voyage - it symbolises nearing the end of a journey. This kind of symbolism is very important in Charlotte Smith's writings. It is also a notorious spot for suicides, another association with death.
I think this poem is the poet, knowing she is approaching death, looking back over her life. Her mind wanders over many aspects of that life, sometimes in a seemingly haphazard way but she ultimately comes back to the inevitability of death.