I will not promote: What's the hardest lesson you learned about selling into healthcare?

I'm a 23-year-old founder building in healthcare, and over the last year I've gone from thinking the hardest part would be building a product to realizing the hardest part might actually be getting healthcare organizations to adopt anything new. I've spent the past year talking with providers, refining the product, walking into practices, getting meetings, and learning how incredibly complex healthcare decision-making can be. What I'm struggling to understand now isn't necessarily product development—it's everything that comes after. For founders who have built in healthcare: What was the biggest misconception you had before entering the industry? When did you know you had product-market fit instead of people simply being polite? Who actually became your buyer? Physicians, administrators, office managers, health systems, someone else? What ultimately got your first paying customer? If you were starting over today with no network, what would you do differently? I'm not looking for generic startup advice. I'd really like to hear from founders who have experienced the realities of selling into healthcare. Thank you!