Hi Anna,
Your experience sounds so much like mine when I was trying to master the basic skills, we could be the same person!!

I battled mask stuff and had to try a few times before I really executed the CESA well.
Just a couple of weeks ago, my husband and I went through a refresher because we haven't been in the water for awhile and at the close of the pool session, the instructor told us she'd dive with either of us anytime. We easily demonstrated that we could look after ourselves and each other. Now, I'm not a very experienced diver... certified in 2001 but with ony about 30 dives in my log, so her words mean a great deal to me.
I was successful in overcoming the skills problems and I feel sure you will be too.
Here are some things that helped me.
Remember that as long as you can breathe through your mouth (with reg or snorkle) there is no crisis.
When flooding the mask, I generally do so by edging one finger under the skirt at the side of the mask. That lets me control the amount of water that comes in and the speed.
When actually clearing your mask, remember to look UP.
Tilt your head back so the water is sitting at the bottom of your mask resting against the skirt and your upper lip.
Then use either or both hands and press firmly on the top of the mask, to hold the top of the skirt against your forehead. This forces the air you exhale to bubble out the bottom, taking most of the water with it. If the mask is too tight, this is harder to do than with one that really fits correctly.
If there's some water left in the mask, inhale again through your mouth, tip your head back, hand to forehead and exhale again. Slow and steady, no crisis, remember?
It's not a race and it's not the ability to clear in one breath that's important. Being able to clear the mask is the goal... whether it takes one breath or several.
Once you have the confidence in your ability to clear your mask of water, you'll also be able to know that if you remove your mask, you'll be able to get it back on and clear. I always get a little water in my nose during this skill, and the trick there is to continously exhale lightly through your nose while the mask is off, just a very tiny stream of bubbles. As long as you can breathe through your mouth, there is no crisis.
I was so glad, on my first dive after cert, that I'd taken the time to really master both these skills during training. There was an overeager photographer in my dive group and as she swooped down in front of me, her fin caught my mask. It ended up off my face, but since I'd practiced this skill until it was second nature, I was able to retrieve it, replace it and clear it. She also managed to partially dislodge my reg (and never even noticed what she'd done.) No muss, no fuss I reseated it in my mouth... and now when another diver is too close, I put up one hand to deflect the fins<G>.
Be patient with yourself and if the instructor isn't helpful in coaching you to overcome the problem, it's time to discuss another instructor.
I look forward to your celebratory post about getting into open water for your first ocean dive. It is an experience like nothing else and so worth the training difficulties!
Welcome to the Board and welcome to diving.
Ginger