Am I wrong for being upset about this?

If you could buy a thick blanket made in South Korea it would help to keep you warm when your room is cold. I am getting a couple of them.

Get your own place if you're over 18.

electric blanket 🧧

Get an electric blanket.

Youre over 18 right ? Then you are entitled to your own space , if your parents wont give it to you then move out

My heated mattress pad is a lifesaver for that type of situation

Tell her to sleep downstairs on the sofa

If possible make a deal , you can turn on heater on alternative day or something like that.

If not, start finding ways for you to live through the current situation.

Also you're not Wrong for being upset.

Hey guys, this weekend or next week we should be looking at houses. We found a perfect house so I'll not only have my own room but my own bathroom as well:D

But I'll try not to get too excited

Is there any space which you could convert to your own little room? Even a walk in closet. My cousin used a walk in closet as his room until he was 18.

An attic, a basement ( even if you have to share the space with storage item)

If not you need to compromise which does not mean only giving in. If its cold one night she can have no heat but the next night that it is cold you can turn on heat.
Buy a mini space heater. The really small ones plug in like a night light and cost less than 20$ and honestly they only heat up a few feet in front of them. I have one teeny tiny one and one medium one that sits on the floor but the medium one was about 40$ and its capable of somewhat heating a whole room or at least taking the chill off.

What is the temperature in your room without the heater?

get an electric bed blanket

Unfortunately, what people say about getting your own place is the only real solution. Unless your parents have a separate room in their house that they’re willing to rent you, it’s not technically a roommate situation. Could you afford to live as a roommate with friends ?