You simply don't KNOW if it's possible for the Ukraine to retake territory or not. That's because nobody, not Russia, not the West, not the Ukrainians themselves can be sure. It depends on the state of the two armies. This is a war of attrition now, and it doesn't matter all that much who takes what village or minor town in the Donbass. What matters is what happens to the two armies. If your army suffers critical losses, even while winning, you can quickly be driven from whatever gains you've made. That's what happened to the Germans in WWII.
For Ukraine, it is critical to come out of this war with as much of Kherson Oblast in their hands as possible. A fortified Kherson Oblast will make it MUCH harder for Putin to repeat his invasion in another 7 years, which most people would agree he is likely to try if he can. All of you calling for immediate concessions and peace need to recognize that handing the Russians what they want will likely only lead to a respite, because they will believe they can get EVEN MORE in another round of fighting later.
If the Ukrainians can reverse some of the major Russian gains outside the Donbass, there is likely a MUCH better chance for a lasting peace.
I include a podcast featuring Michael Kofmann, a true expert on Russia and Ukraine, discussing why this might be Ukraine's window of opportunity coming up:
https://warontherocks.com/2022/07/ukraines-window-of-opportunity/