Since your pressure is fairly high it becomes even more important that both your face be oil-free as well as the mask cushion, if only one gets cleaned then the other will become oily and that's an open invitation for mask leaks.
A silicone cushion won't stick to your face if you apply moisturizer/cream either.
There's also a possibility you may not have the correct size mask, you said the nice woman sent it to you but, did you try-on any other sizes while you were at the DME? A lot of people don't match the ResMed sizing guide for whichever mask they're trying to size and sometimes you have to just try a different size before finding one that's just right.
Just trying to cover a few more possibilities.