The first week is always the most intensive in any war. It eventually dies down in intensity. Also, we more or less know the numbers fighting now on the Russian side, and it's not a very impressive number. And 1/3rd of 150-190k sounds impressive, but 1/3rd of way less than 100k does not, especially without knowing the extent of Ukraine's losses.
Russia took most of its casualties in the second, 3rd and 4th week, when they were sitting like ducks waiting for resupply. Russians raced to Kiev without clearing the countryside. They got absolutely demolished in the 2nd, 3rd and 4th weeks by the Ukrainian ambush units still controlling the countryside.
As for the Russians retreating, that's only Ukraine's side of the story. Two days ago, the Russians took 3 villages east of Izyum. The media on both sides are not giving us a complete picture of what's happening on the ground. Outside Russia, Ukraine is winning the perception war, the most irrelevant aspect of the war when it doesn't conform to ground realities. But, at the same time, it's the Ukrainians and the West pushing for talks, the Russians don't really seem to care at all. So that doesn't match up to reality.
Russians are moving forward like snails while retreating (running away) hundreds of kilometres. I am surprised that you used Izium as an example of Russian advance

.
In Izyum, Russians are fighting for more than 1.5 month and they can't move forward any significant way. Russians advanced a grant tottal of 5 kilometers south of Izium in 1 whole month. In that process, they lost thousand of troops.
Map of Izium front April 18th, 2022

Map of Izium front 1 month after (May 17th). Russians made an advance of 5km in a whole month.
With their current strength I doubt they will be able to take all of the Donbas, or even encircle the Ukrainians from Izyum to Donetsk, but neither the published manpower losses nor the media perception match reality. To make matters worse for the Ukrainians, the Russians are busy digging in, in the West, Kherson and Zapo, while the offensive is happening in Donbas. And a new offensive to take Severodonetsk is building up, especially with Russia making gains around Izyum, Lyman and Pospana. It's believed there are 20+ BTGs in Sever, about 10 in Pospana and 20+ more in the Izyum/Lyman regions.
Simply put, the Ukrainians are unable to fully stop the Russian offensive, where the Russians want to conduct an offensive, no matter how slowly it happens. And wherever the Russians have retreated is being considered a victory through a counteroffensive, just 'cause the Russians have retreated into more defensible positions in areas where offensives are unnecessary at this time.
Ukrainians already stopped Russian offensive. Or better, they counter attacked and recaptured ( Or as Russians call it, glorious retreat) thousands of square kilometres. "
Retreating from areas where offensive is unnecessary at this time?"



. Did you mean areas where Russian offensive failed and got absolutely demolished?
Russian attack came to a halt in all fronts and they are incapable of concentrating forces for an attack, let alone conduct a successful offensive.
At this point of war, daily Russian loss is similar to Ukranian losses or even higher. Ukrainian Artillery units proven to be far more effective than Russian one.
Ukraine is fighting a total war with full mobilisation. They can afford to loose more troops, Russia cannot. And Russia not asking for ceasefire doesn't mean Russia is winning. Temporary ceasefire favours Ukraine,that's why Russia not accepting it. Temporary ceasefire means more NATO trained troops and weapons, more repaired bridges and more time to bring heavy weapons to east from the west. Russia gain nothing from a ceasefire, so they obviously rejecting it.