Green’s Dictionary of Slang

milkman’s horse n.

[rhy. sl. = Cockney pron. ‘crorss’]

adversity; an emotional burden.

[UK]G. Kersh Fowlers End (2001) 268: A tosser on a Wilkie Bard, / A lord on a Charing Cross, / Is ’ow I fell, and it’s bread-’n-lard / To bear my milkman’s ’orse.