My daughter was terrified of the mask remove and replace, and I did a lot of work with her in our backyard pool between her certification pool sessions. Keeping her head as close to the surface as possible really seemed to help (top of the head was at the surface).
We worked from clearing very small mask leaks, working to a flood below eye level, and eventually a full flood.
Once she was comfortable with that (a couple of pool sessions), we moved on to the mask remove and replace, but started with a very rudimentary "remove." We just kept he strap a bit on the loose side, so the entire mask could be pulled forward off her face and rapidly flood the space. Then replace the mask and clear like she was used to. Fairly quickly, we got to the full remove. As she got more comfortable with it, we also progressed to where instead of having her head touching the surface, she was kneeling on the bottom in 4 feet of water. Her head was fully submerged at this point.
A couple of days later, she had it with comfort, and could remove her mask and sit there for 2 or 3 minutes until I tapped her arm to replace the mask, which she did. I knew she had it when her hair got tangled in the mask buckle. Instead of panicking, she stopped, thought it through, and then yanked as hard as she could. She pulled a good sized chunk of hair out of her scalp, but never panicked and completed the exercise. It hurt just watching it...
The key here was just taking very tiny baby steps, starting at the surface, and with tiny amounts of water in the mask. We then progressed in water volume and to being able to do it fully submerged. It did take a few extra pool sessions though.