> What attempts at independence has Ukraine made "for hundred years" which Russia suppressed?

1917–1922, more here https://www.wikiwand.com/en/articles/History_of_Ukraine

> Half of Ukraine is ethnically Russian and speaks Russian

Usual stuff empire does is moving their loyal people to colony to mix population and make it more loyal. For example after Crimea annexation russia moved hundred thousands russian to crimea, specifically military people and their families. 

eastern part of Ukraine has a lot of factories built during USSR and because of that lots of russians moved to eastern ukraine. specifically to big cities, if you were in Ukraine you know that the moment you left big city, like Kharkiv, you see a lot of ukranian speeking people.