Can I still sue if I signed a document saying I was satisfied with my due diligence?

Generally yes, in the right circumstances, even though it can feel counterintuitive. A clause where you confirm satisfaction with your own due diligence is usually meant to stop you from later claiming you simply didn't look hard enough, or that the seller had some broader duty to volunteer information beyond what was actually asked — it's aimed at your own diligence effort, not typically meant to shield a seller who made an affirmatively false representation in the purchase agreement itself.

The key distinction is between your own investigation (which that clause addresses) and the seller's separate, ongoing representations about the business, which remain a distinct promise regardless of how thoroughly you dug in beforehand. A seller who represented something as true that wasn't generally can't hide behind your due-diligence acknowledgment simply because you didn't happen to catch the falsehood yourself. Whether your specific situation falls on the "should have caught it" side or the "seller misrepresented a specific fact" side depends closely on the exact clause wording and what was actually represented.