Good luck! I am also what I refer to as a "Cinderella" with boots, having gone through 5 pairs in 6 seasons (more if you count the pair I had when I moved to Utah!) I have super low-volume feet and hypermobile feet and ankles. It's hard to feel secure with such issues, but I will go back to what
@KingGrump said earlier about a technique issue (this is definitely part of the equation for me) and what
@HardDaysNight says above-get the heel and ankle locked down and a slightly larger toe box can be worked with.
MY personal preference is for boots with taller cuffs. Of the 5 pair, I've had two with taller cuffs-a Salomon XMax 90 and now a Nordica Promachine 105. The cuff on the Promachine (with the catches moved to the narrow side) are incredibly snug. I think that my narrow, loosy goosey feet like the extra control I feel from a taller cuff. These two were and are my favorite boots by far. The Xmax became too soft and also just had too much volume.
So, I feel for you and wish you luck. Sometimes, it's the equipment, sometimes, it's the skier, but sometimes it really is both. Also, some of us are super sensitive to what is going on around us, under our feet, on our feet and it adds another layer of frustration to the situation. I get the impression you could fall into this category.
Video of you skiing would be great! Also, don't be afraid to ask for instructor recommendations here and get a lesson.