Hi Alexandra! It is nice to hear from you. First let me say that you are doing a great job on this piece. Your issues aside, it’s looks great so far—Bravo!
As you probably are already aware, our perception of how “light” a particular material appears depends on a number of factors. From application to context–the lightness we seek can sometimes seem frustratingly elusive.
Let’s first look at two contextual factors you may want to consider. Issues with material reflectance properties and photography aside, I compared your drawing with your reference source by sample the source and moving it to the corresponding piece of your drawing. With this comparison of samples (in the circles on the right), you can see that the halftones are slightly lighter while the highlights are considerably darker. Now while it won’t change the actual reflectance properties of the white highlights, you can push your surrounding halftones a bit darker (indeed allowable by what the sample comparison shows) and you should perceive the highlights as being a bit lighter. You can see this effect in the demonstration on the lower left here. each chevron is identical–however the context from the background significantly alters how we perceive them. The second contextual factor, related to the first, is how the manner in which one value appears to transition to its surround may impact the perception of that value. For example, on the lower right, we see a gradation surrounding the central circles in such a way so as to make them appear lighter/brighter than the same white found in the surrounding striped pattern.
Next I would have you consider your manner of application. With many dry materials that can be regarded as relatively “hard”, a heavy hand or hasty application can quickly impose an unwanted ceiling on your level of perceived lightness. I would recommend watching this video that address some of these factors:
https://www.smartermarx.com/t/drawing-video-layering-dynamics-of-hard-white-pastel/60
I hope this information helps! Let us know how it turns out~~~ 