former Baltic republic were a part of USSR so they were like Russia
You don't know anything about the USSR, do you?
Greatly simplified version.
Basically it was the russian empire under a different name, russians were the dominant ethnic group and mostly liked the USSR. Most other ethnic groups saw the USSR as a prison.
In the case of Ukraine, Ukrainian values are very different from russian values. Ideas like freedom, individual rights, human dignity and the value of human life are mostly non-existent in russian culture while they're more important to Ukrainians.
russians never want to stand up against a government they don't like (preferring to suffer in silence) while Ukrainians don't mind getting in the government's face. Poland and Ukrainian are more similar than different in that regard.
Belarus is a bit different.... Poles mostly get along with younger more educated Belarusians but the governments are enemies (since the repression unleashed by Lukashenka after the fake elections.
The single most important factor uniting Poland, Ukraine, and the Baltics (not Belarus) is strong dislike of russian government due to various forms of russian repression in the past.