You know, the ski instructor told me the same thing and the guy at the ski store said, "You can't listen to those instructors."
OMG! Seriously??? What an a$$hat. Always give more weight to the opinion of someone who has
actually seen you ski. Even over what you read here. 'Cuz here's the thing- we can suggest things to look at or to try, but we can only give you feedback based on what you tell us. We can't see you ski to get a more objective view of the problem (that said, even instructors have their own biases.) And Shop Dude is even worse. He seems determined to hear everything you say through HIS incredibly biased (dare I say, sexist?) filter. Caveat: I have learned to ask much better questions, and therefore, get much better answers, from the wonderful advice & suggestions I have received from other Divas. I have also learned how to more effectively describe a problem I am having and get assistance with said problem!
I don't know the degree of training of your instructor. I will say that that could be a variable as well. If you got an instructor like me, who is still new to instructing, then yeah, maybe you need to take their suggestions on the right ski for you with a grain of salt. From the way you describe your skiing however, I highly doubt you would have gotten a new instructor. Was your instructor PSIA-certified? Do you know to what level? It can take YEARS of training and hard work to get to Level 2 or 3 certification and along the way, you do have to take classes to learn how best to teach people, how to diagnosis their movement issues and ways to begin to fix/address it. I HIGHLY doubt Shop Dude has had any such training...
So there you go, don't trust Shop Dude.
Also, read some of the other threads around here, especially that Kim Reichlem one I mentioned earlier. Seriously, its an epidemic of Shop Dudes arrogantly sticking ladies on skis too short for them. That thread is FULL of stories of other Divas who know they ski much better on a longer ski then is 'recommended.'
I would also second
@Pequenita's suggestion that their might be a lurking technique issue. I already communicated the story of my DH's bad day on the new skis last week. They were 10cm longer, probably have a crappy tune, but also have traditional camber and no rocker whatsoever. With the shorter 150cm skis, he was able to smooth over some of technique issues- old habits from BITD when he skied the straight skis. The 160s skis did not let him get away with this. Granted, there are some other variables at play here, but my point is even a traditionally cambered ski might not solve all of the issues you're having. Ideally, I want to get him a ski with at least an early-rise tip for easier turn initiation. Probably something around ~160 so that its a little longer than the 150, but not quite as unwieldy as the 160, for him. (As an aside, ladies have a hard time finding skis long enough for them- my poor hubby has the opposite problem, it seems they don't make men's skis in shorter lengths! I might end up getting him a woman's ski, just to have more to choose from!) Many of the Divas here have mentioned that the 'ski length calculators' can be just plain wrong, particularly for women who are advanced skiers, tall for their weight or heavy for their height (all that muscle weights a lot!!) So now I'm beginning to wonder about the lengths of the rentals you have used in the past. Its entirely possible that the folks at the rental shop are using one of these calculators and have been giving you skis too short for you all this time. And because they are too short, you are able to overcome some technique issues with them anyway. Just another thought...