When you’re drawing a subject from observation, don’t try to look at it at the same time as when you’re looking at your sketchbook. Look directly at your subject, take a mental snapshot of a part you want to draw, like an edge or a curve, then try to capture it. Look back and compare. It doesn't need to match perfectly, it just needs to get the right idea across. If you captured it accurately, great, move on to then next part. If not, adjust it until what you have on the paper captures what’s in front of you. Don’t stress out about the tiniest of details. It takes a lot of practice to develop an eye for accuracy, so just keep going and you’ll improve. Hope this helps :)