if I get tested positive for covid the day before I fly, how soon can I take another covid test?
First and foremost,
if you tested positive for COVID and are experiencing symptoms, you should absolutely not travel. The CDC guidance states: "Stay home for 5 days. If you have no symptoms or your symptoms are resolving after 5 days, you can leave your house."
If you are still experiencing symptoms on Day 5, your quarantine is NOT over and you should not travel until symptoms subside.
I also heard about the you could get a note from a doctor to excuse you from it. Can someone explain to me how that works?
If you did have COVID, it's likely tests for a while will show up as positive - which is why you need to hold on to the Positive Test. You need to see a doctor in Mexico to get "a letter from a licensed healthcare provider or a public health official stating that you were cleared to travel." If you can't find a doctor, you
will need a negative test. But tests are unlikely to "turn" negative in the month after you have COVID.
Do I need to buy insurance?
Some Travel insurances cover a positive COVID test - I recommend you read their Terms and Conditions VERY carefully. I imagine if they do cover inability to travel due to a positive COVID test, your test will have to be dated after the flight was booked.
So if I book Jetblue Blue... I could cancel my flight and get ecredit or rebook right?
As for JetBlue fares, you will not be allowed to take a carry-on with a Blue Basic fare. You will need to book a Blue Fare (or you can book a Blue Basic Fare with a checked bag or a Blue Basic with Even More Space seat). Blue fares also allow for free cancellation. Any cancellations will refund the fare into JetBlue TravelBank (which will have to be spent on JetBlue Travel in the future). Because JetBlue Blue fares allow free cancellation, there's no real reason to buy insurance IMO.
According to the guidance, technically, yes, you can travel if you get a negative test. In your situation, it does sound irresponsible to travel given your positive test and experience of symptoms.