"Beatland" - What does this word mean?

The context is this: "What can balance the inequity of that huge space, which never ends, and my bounded life? Perhaps this: The beatland of my body is not my kingdom's scope, I have within, spaces as vast, if I could claim them."

Before saying this the writer talks about how vast and unknowable the stars are, how death and infinity can't be known. So, I agree that beatland means borders, and the writer is saying that finite (bounded) life of the body contains space as vast as the milky way. I think the boundaries refer to physical boundaries and the limits of time -- length of life. Which nevertheless can contain infinity.

Since she also uses 'Kingdom', we can tell that she is using a term about the extent of land in a metaphorical sense to discuss herself and her body. Given that, it is likely that 'beatland' is also a term to do with the extent of territory and we can make the leap to the custom of 'Beating the Bounds'

Beating the bounds is an ancient custom still observed in some English and Welsh parishes. Under the name of the Gangdays the custom of going a-ganging was kept before the Norman Conquest.1 A group of old and young members of the community would walk the boundaries of the parish, usually led by the parish priest and church officials, to share the knowledge of where they lay, and to pray for protection and blessings for the lands.

While the word 'beatland' doesn't seem to appear much in this context online, a closely related usage is attested in this example from Dorset. Beatland Corner Socket Stone

Waymarker on the ancient route taken by the monks between the Augustinian Priory at Plympton and Tavistock Abbey

A likely definition of 'beatland' would then be 'the land within the Parish Bounds'. Winterston would in that case be making some comparison between a parish and a kingdom, but without more context I would not hazard quite what the comparison might be.