no worries, if you read my other comment you will be fine. It sucks you didn’t get your tourist visa but as an American you are eligible for the visa exemption on arrival. You can get on your flight without a problem. Do make sure your passport is at least valid for another 6 months though.
I believe you are American with an American passport? Then you don’t need a tourist visa to enter Thailand. You are eligible for a visa exemption on arrival. This will give you a 45 day visa exemption stamp on arrival at the airport which can be extended with another 30 days at the local immigration office here in Thailand. If you are planning to stay in Thailand 75 days or less, you will be fine.
if you crash or hit someone and they have to go to the hospital will your insurance still cover it, since you’re obviously driving without a valid Thai license?