You can control a bike with a freewheel but you must cooperate with a fixed-gear bike.

Sorry to be philosophical but it takes a while to learn not to fight the bike. You have to ride it on its own terms. The more you ride, the more it will become easier.

One very practical tip is to not pull too hard on the bars while climbing. My oblique muscles and forearms ached when I first started riding fixed on hills, from pulling on the bars too much.