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I'm sorry to hear about your situation with your attorney. Essentially, you got an illegal sentence and your attorney didn't notice, which is a pretty strong argument in favor of ineffective assistance of counsel. Typically, it is hard to make out an ineffective assistance claim, because the court gives a lot of deference to attorneys' trial and strategy decisions, to prevent someone from just saying in hindsight that they didn't like the strategy their lawyer took because it didn't end up working out. However, failing to notice that you received an illegal sentence is not a strategy decision...it is textbook ineffective and you were obviously harmed by it.
I would suggest reading over this case. It is the main ineffective assistance of counsel case in the country where the Supreme Court outlines what it is:
In addition, the fact that the sentence was illegal can be grounds for a 2255 motion by itself, without even proving ineffective assistance of counsel. So in your motion, you can argue that 1. your sentence was just straight illegal because of the fact that the requirements for the mandatory minimum were not met, and 2. your counsel was ineffective for not raising it.