It is not impossible. I have only done it on occasion but that was mostly due to lack of attention span rather than poor playing. I find my biggest problem as I progressed was that I only was focusing on a few quality troop producing provinces while everything else was for commerce or left to rot. As you get more and more land you need more troops so focus on building up infrastructure and pump out troops as much as possible dont stop!

Oh and a navy moving troops quickly is key

One more thing Tax rates and influence I suspect by 70% you can afford to set the "Very Low" option if so do it, and make sure your King isnt some weak kneed yes man...Get those crowns up!

This help?