Russia has the Donbass separatist militias, which would make for a perfect occupation force - they are ideologically motivated, but also, they know that they'll be treated as traitors by any Ukrainian state with more than a semblance of independence.
Many of those 44 million will become refugees in Europe. And some - not many, but probably at least a couple million more - will support the occupying forces.
Authoritarian regimes manage to keep their population in check with forces smaller than that.
