Depends on your flight arrival and departure I.e. 2300 arrival time stay airline until 0000 then go through, you gain 1 day. Do the same on the way out you lose a day, then you have stayed 30 days, that is if your from an EU country that gives you the 30 day VOA. Failing that go to immigration and get an extension.
If your a country with less than 30 days VOA then you have to ask again. Bon voyage
Review your booking, it will tell you all fees including cancellation. Most are not aware the majority you pay is taxes and in most cases you are entitled to those taxes to be refunded. That's if you read the small print in the first place as some do not offer any change or refund.
If you can get a tax refund cancel the flight and book a new one. If not stay as is or put it down to experience, book and pay for a new one. An airline will not help, you booked through an agent and you must use that agent.
It's not just Thai partners, things have tightened up, marriage is no guarantee, I know someone who was an invalid and even he struggled to get his now wife in.