Getting Ukraine to cede territory is going to be an uphill battle because the last time they did it, our security guarantees meant jack squat. Also Russia is never going to let them join NATO because they fully intend on taking the rest of Ukraine. The only way to keep them out of NATO is to continue the war with them.

So, if Ukraine cedes the territory, they have just legitimized the Russian claim to that territory while getting nothing in return. This is why talks have always fallen through up to this point. Russia wants Ukraine, Ukraine does not want to be Russian. There is no middle ground.