The Flow Binding Size Chart shows that with an 11.5 size boot, you should be in an XL Binding. However, this is a guideline and your low profile boot may well fit better in a Large size binding. I am in a older Burton Tribune boot, size 11 and in a Large Binding and the fit is fab. I know that last thing you want to hear is "wrong size" but I would seriously consider it as your biggest reason for your fit issue.
Watch the video above and check weather your highback & cable are in the forward or backward position. Given your boot size, I would think you should be in the forward position.