As above make sure it’s the right mask for you. Can you put it against your face without the strap and inhale through your nose and hold it on your face with just the nose breath.
A mask is so important. Clearing a mask is not that easy and it’s something I used to practice in the pool sat on the bottom for hours.
Your mask shouldn’t just fill up unless your smiling or laughing that gets mine full.
Just practice but if it fills up on its own it may be too tight or just the wrong mask for you.