I have tried several full face masks and the seal keeps breaking.
I have also tried the nasal pillow, but my jaw keeps droping open.
I have a dental guard for sleep apnea, but my oxygen level still falls when I wear it.
It's a real struggle for most of CPAP users to find a good CPAP mask fit.
If I would know more about the type of CPAP mask you're using, then I could give specific information to help you getting a better fit.
Without information about your CPAP mask type, I can only offer you general tips to improve the leaks, such as:
starting with choosing the CPAP mask in DME shop, the supplier should help you choosing the correct size mask. The mask should be tested inside the shop, with your CPAP machine running.
test the mask while you're lying down, like you're sleeping.
don't tighten the straps too hard. In fact, should be tested with a very loosely strap-tightening technique, then you tighten the straps gradually until you get the perfect fit.
facial oils can negatively affect the mask seal. Washing the face prior to applying the CPAP mask may be helpful.
you can improve the CPAP mask seal and your comfort level by using RemZzzs CPAP Mask Liners . But you have to buy them in the same size and type as your CPAP mask size.
make sure you're placing the mask where it's intended to go. For example, the Quattro fits on the bridge of the nose, but the Fisher and Paykel Flexifit masks goes high up on the nose, between the eyes.
use a chin strap to help you hold the jaw in place so that the mouth does not fall open. Even if your mouth is covered by the full face mask, you can get less pressure into your airway which needs a high pressure to be kept open.
By opening your mouth during sleep, it will also causes your face to change shape, which increases the leak rates.
So your mouth should stay closed while you sleep, and the chin strap will help you do that.
I would also add a video where you can learn more about how to improve the fit of CPAP full face masks: