Greg_Kat least grab a couple pieces of paper larger than your feet and trace out both feet without socks and then measure the length and width in mm. If it’s 295mm, that’s a 29.5 boot and the width is the Last.
Nah, almost all 29.5 boots have bsl's between 330-339.
OP, ski boots are far and away the most important part of your setup, as they are the only hardgood that comes in direct contact with your body and they're how you tell your skis what you want them to do. Ski boots are designed to be containment devices, they work best when your foot is fully locked in.
Fit wise, when you stand up straight, your toes should feel the front of the boot, maybe even curl a little, but don't worry. You're not gonna be skiing in em like that. When you drop into that attack stance and put pressure on the tongues and flex the boot forward, your toes should back off the front of the boot and have a little room to wiggle up and down. Your heel should be locked in, no side to side slop or front to back sliding. You know what a firm handshake feels like? That's how you want your boots to fit.
Buckling wise, first you want to put your foot into the boot, get it situated, then flex forward a couple times to get your heel to seat snugly in the heel pocket. Once your heel is nice and locked in back there, get the second highest buckle on your shin pretty damn snug. The top buckle should be almost as tight, but that second highest buckle is the most important. For the bottom two buckles over your instep, you want them pretty lightly buckled - you should be able to close the buckle with the pressure of one finger, they exist pr much just to keep the snow out. If you have to crank those down to get the boot to fit, there is too much volume in there and you're in the wrong boot to begin with, and your feet will almost certainly get cold/go numb because you are restricting all that blood flowing to your toes over your instep.
In addition to being in too big rental boots, you're probably skiing in the backseat and your feet are sliding all over the place in your boots, your shins are banging into and off the tongue, etc. Like earlier people have said, try to always keep your shins pressured against the tongues of your boots, and be in an athletic stance like you're about to tackle someone - legs shoulder width apart, knees bent, weight forward, hands out in front of you. Hope that helps!