Insurance is very difficult for pre-existing conditions and have many terms and conditions, such as the condition being stable for many months with no changes to medication or any doctor/hospital emergency visits due to the condition. And any insurance you got would normally just cover future unexpected emergencies, NOT your day to day medicine or regular checkups.
As for paying, this is up to the place you go for treatments. A hospital or doctor may demand you pay cash or deposit upfront and deal with insurance company yourself, it's entirely up to them.