Consider the root cause of feeling the heat is fat. Fat is an insulator keeping your heat in your body. It adds weight which adds strain to everything you do and because of this puts added stress on your body's ability to cool itself. This all occurs outside of a hot car on your own, so when you get into one, you are adding even more external heat to your core, reducing air flow which further restricts evaporative cooling making you even hotter. I imagine this can induce some serious fight or flight response to get out that environment.
Everyone has already hit the head on the nail, with weight reduction. Some folks have glandular disorders like hashimotos disease or PCOS which makes losing significant weight almost impossible even with hormone correction so it is important not to default to "you're a fatty and need to stop eating donuts and get on a treadmill" LOL. The solution is not always that obvious or straightforward.
Anyways, I live in the desert and as such cannot stress enough the importance of a fully charged and up to snuff AC whether you're skinny or fat - doesnt matter. As already stated, if your AC has not been serviced, get it tuned up so it is blowing ice cold air and lots of it. Also, as already suggested, run your car with the AC going for a minute or two before you get in. This is AFTER you have opened your back door and then rapidly open and close your front door a few times, This will "flush" the hottest air out the back and make it easier for your AC to cool your cabin interior thus when you get into the car, it feels better.
If you have a big belly, when you sit down in the car seat, especially if its a small car, your belly pushes up into your diaphragm making taking full breaths of air harder or sometimes impossible which is also panic inducing. To help counter this, if you can, pull your car seat as far back as possible and tilt the back of your seat back enough to reduce that pressure on the diaphragm muscles. It will help you to be able to take bigger breaths which in a hot car is really important.
Now if your AC is broken and you cant afford to fix it (Ive been here a few times myself) this will sound silly, but drive faster with all the windows down. If you can, wet your Tshirt a bit before you get into the car or even better yet, get it wet and put it in the freezer for while. When you drive fast with the windows down, the blowing air across your wet shirt will improve evaporative cooling, whicking away heat from your body and if you can chill your shirt in the freezer, it cools your core outright helping delay over heating.
Another thing I have done is fill a hot water bottle with ice and water and stick that between my legs or across my gut or across the back of my neck LOL. I did this when my AC broke and it was 103F outside lol. Driving around was not pleasant and this really helped.
Along with this I highly recommend loose clothing. If you can, ignore ones modesty and embrace sleeveless tank tops, shorts, sandles, etc. I am lucky in that all my hair fell out, but consider shaving your head during the summer months or at least keep it SHORT.
In the weight loss category, I myself have been using Wegovy to help me lose/manage weight in my late 40s. It has really lessened my hunger and I have lost about 30 pounds with it and naturally eating less because Im not as hungry all the time and moderate but consistent physicial activity. What do I mean by that? I take my dog on walks, take my kids on bike rides, rake in dirt and over seed on my grass, putz around in my garden beds, go hike with the kids in the mountains, go birdwatching etc. Im not into "working out" or sweating much (Im not a sadist lol), so for me its more about doing the stuff I like to do that is already fun or enjoyable for me.
Anywyas, best of luck to you. I hope you feel better
