Maybe this specific to left-handed-artists (me) because we spend most of our lives translating from a right-hand world. But I discovered I was working on an image that was facing the opposite direction from my reference photo. Turns out to be a completely different picture than I expected and I could not figure out why for a few moments.
Don't "practice" on your painting. Start each session with a few strokes and color experiments on a similar piece of paper (scrap) with colors you might not have thought would work in the painting. This will get your hand, eye, brain and attention aligned and ready for the real thing. You might even make a few exciting discoveries too. Don't forget to make notes!