Allowing plans to sell across state lines would have virtually no effect because the cost of building a provider network would be prohibitive. Six states have enacted laws to allow it, yet not a single new insurer has entered their markets.
If you live in Florida, the risk pool is already substantial enough. Expanding it nationally would have no meaningful impact on your rates. Not sure what point you're trying to make about Medicare.
I think
@ticket2ride04 already pointed it out, but you don't have to buy a health insurance policy on the exchange. You could simply buy an individual plan from one of the numerous brokers in Florida. I'm also a little skeptical about a doctor not accepting your plan. Which insurer issued it?