Try widening your feet a bit, as far as shoulder width. Also focus on keeping your weight on your front foot, using the rear just to balance. And do it next to a wall or chair or something to hold on to until you get better at them.
I found that elevating the back foot on a weight bench made it a little easier to balance because I was more stable and forced to stay over my front foot.
You could also elevate the front foot on stairs, or a plyo box, or a stack of weight plates/yoga mats, or whatever else is available.