I’m sorry to hear that your first attempt to fix your problem didn’t exactly work as planned. Luckily, the answer may be quite simple.
If your car is still overheating even after you’ve changed the thermostat, you’re likely dealing with an issue with your radiator. It could be clogged, which would restrict the flow of coolant and cause your radiator to overheat.
The best thing you can do is take your car to a mechanic - perhaps a different mechanic will be able to assess the situation a bit better.
If your vehicle is under warranty, your dealership might also be able to help with the situation, and it may even be completely covered!