Lovely painting! Have you been fixing the layers with fixative between applications? In my experience of working in pastel on stretched canvas using Colorfix primer, I have always varnished it with Acrylic varnish. One coat of gloss and then two of matte. I brush it on, but it has to be fixed really well other wise it smears. I have known other artists to use thinned down acrylic medium in a sprayer, sometimes between the layers, and I think some have used a spray varnish such as Golden UV spray varnish. All darken the painting quite a bit.
Is this your first painting executed in this way? If it is, I would strongly suggest that you do some tests on small canvas paintings first to test out the possibilities of sealing. Acrylic varnish does change the nature of the finished pastel, and makes it look more like an oil/acrylic painting.
There is an archived article by Sandra Jackoboice in the Pastel Journal….(If you go to either the Pastel Journal’s web site, or hers, there is a link to a PDF file) …which might be interesting for you to read ….about her techniques with pastels on canvas.
Please. Please, do some tests first so that you don’t ruin your nice painting.