I also had some difficulties when I first started doing the Aura of Protection and Aura Cleaning meditations. What fixed the issues were, first, visualizing myself from the third perspective and, visualizing the aura/energy like I would see it in a fantasy film, or even animation.
The texture, the intensity, the shape, every aspect can come from something you have seen already, and that makes it very easy to visualize and sustain the image.
You can experiment and change some aspects to see how well they work. I noticed that if I switch to another style of representation once every week or so, I retain focus. Maybe it's because I don't let myself get bored with the image. I'm very prone to losing interest if things become samey, and this method works very well.