It's really hard to see that Ukraine by itself could defeat Russia and it definitely seems far fetched, even if the Russian army has shown itself more inept than expected.
But that isn't what is happening either, and a Ukraine bankrolled and armed by the west. That is something else, and that combination might definitely be abel to defeat Russia. So the question is more, is the west or parts of it willing to spend enough and accept escalating tensions with nuclear armed Russia by doing that?
If you look at what Zelensky is doing at the drumbeat of asks for more weapons is obvious that Ukraine is fully aware of this.