How to keep cream from separating in milk?
Homogenization is the process that breaks down fat particles in milk so that they will not separate.  The least expensive hand-held homogenizer I found on Amazon is over $700US. I would say your options are (a) buy a homogenizer, (b) agitate your milk regularly, (c) buy less milk more often, (d) skim off the cream and use in another way, or (e) ignore the separation.