It’s a war of attrition that Ukraine is losing because they have a much smaller population to draw on and rely on weapons from the west. Ukraine is already conscripting children, women, and the elderly now. It’s absurd to think that such conscripts are going to be able to hold off a seasoned professional army for long.
I recommend reading what people who actually have a clue have to say on the subject https://www.russiamatters.org/analysis/whats-ahead-war-ukraine