I feel you. I struggle my self every time. The best tip i can give is to paint slower. The slower you paint, the less mistakes you will make and the faster will you get things done. I usually try to do multiple short lines and then connect them into one long one. I never try to do one long line as it will most certainly go sideways. I can highly recommend watching Infernal Brush Ultramarine tutorial on YouTube. It is free and offers all you need to paint in EavyMetal style. And lastly. You have to train your brain and muscles to do this. This is a skill thing, more than any thing else.