"I thought the war momentum had always been against Russia. "
Depends on how you look at it.
Russia did invade and took some occupied some territory.
Russia wasn't able to take Kiev.
It took Russia a very long time to take Mariupol.
Russia has paid a very high price for the territory they have occupied. I understand it's normal for an invading force to lose 5 times more than the defenders.
Russia has lost over 3/4ths of the tanks it started with.
It's made only very small gains in the last two months.
Russia still has an advantage in a pure artillary war, with regard to number of artillary units and shells, but the recent delivery of HIMARS is allowing Ukraine to take out Russian ammo depots and HQ's.
Ukraine has cut supply lines brdges and railroads to the Kherson region, trapping 10 to 20,000 Russian soldiers.
Russia has spread it's troops thin in the Eastern part in order to defend Kherson.
Bases are now being attacked in Crimea.
And by transfering the Mariupol prisoners of war back into the Dombass region and then blowing them up. Ukraine's troops now understand surrendering to Russia is a death sentence. So they will fight to the death againist Russia.