I asked my good friend, who had cervical surgery the end of September. H said he had to throw all the pillows he previously used in the closet and find a very squishy one he could "mold" around the c-collar. (He also uses a CPAP, so a mostly back sleeper.) He also said not wearing the collar and doing some stretching exercises in the hour before bed helps him.
Aren't you glad I live among a community of "older" people with just about every health issue you can imagine? There is always someone to ask!

What have you tried so far?
Sue