His motivation was that he thought it would work. He thought he had a great army, he thought the West was weak and divided, he thought Zelenskyy would flee the country. Now his motivation is the sunk cost fallacy.
If he pulls out now having not achieved anything except destroying the Russian economy then his government is surely doomed. It seems likely that he would eventually be killed or at least imprisoned for life after such an event also.
Therefore his only option is to carry on in the hopes of getting to the point where he can claim some kind of victory, however likely or unlikely that may be.
Just to add, he already moved his demands from "Surrender all of Ukraine" to "Surrender those two territories". The one horrible thing here is that there is no demand that he can "win" without harming a lot of innocent people, so he will keep pushing it, no matter how bad things get.
He wasn't far off. It's American stratotankers with British Eurofighters and F35 circling above Poland along the border with Ukraine... the response and sanctions without these two would limp behind if we counted only on continental EU countries, then Nordics only care about their own backyard.