Must be the first time they got good equipment.
I'm not saying this because I'm happy about it, but in the last two weeks , Ukraine lost 5463 soldiers. It's tragic loss of brave men. The last two weeks were the most devastating for Ukraine.
All those western weapons have small impact in reality on the ground. Ukraine found out that Western weapons were fancier and with computerized control unit but in actual high intensity war as it is happening in Ukraine, western weapons tend to break a lot. Ukraine doesn't have the technical knowledge to repair them and they don't have the spare parts. In the short time of training they have been trained just to use them. The much promoted American M777 artillery have a break down rate of 30% in the first day of high intensity use, according to Ukrainians soldiers. Western heavy weapons were designed to be used in low intensity conflicts with partisans in Afghanistan, Iraq and Syria in which the heavy artillery and the multiple rocket launchers were occasionally used. The western militaries are heavy aviation dependent armies. Heavy artillery is Russia's game.
The other consideration is Morale, the Russians and their fighters (Chechnya’s etc.) have their commanders on the front line fighting with them. This is how they operate, Ukraine has been using NATO tactics of not having their commanders with them (This is how the US operates, the strategy being if the commander is killed or capture it's a huge lose, which makes sense) BUT most of the Ukrainian soldiers are not use to this US tactic and see it as abandonment -> hense low moral.
It works for the US military as this is the way they have always done things, but has not worked in Ukraine. This is the same thing (low moral/lack of leadership present) that happened in Afghanistan when the US backed troops lost to the Taliban when the US pulled out and things fell apart.
