The reason that a real corset does that is because it is pushing anything it can to get the waist size you tighten it to. I think although it might cost more, maybe a custom corset maker can help you fight that issue as they will know just how it is made and can make it to your specific body. I myself am getting a custom corset made so I know that it is not cheap. Also are you going for an under bust or over bust? It might put stress on the body in different ways to create the undesired look you are getting.

I really hope this helps even if it is just a little and I hope you find something that you love!