Look there will be a lot of guys on here telling you to run or whatever but you dont need to do that. She is not thinking rationally right now but that is ok. What you need to do is give her some time to calm down. You can say... We can discuss this in A few days when we are both calm then stonewall her after that. For a few days on the subject. Not completely ignore her. Just tell her we will talk about this in a few days. Getting married is really important to some girls. And it can feel like she found out she was pregnant and then told her to go get an abortion or that you slipped abortion pills into her tea. Ya i know that sounds crazy but that is how it is feeling to her. She thought she was going to get to move in with you and get a life together and share income and cuddles on sunpday mornings and have all of the attention from her friends and get a party... She has had 3 weeks to imagine and the happy moments have been unfolding like a movie since then. Its like she has been binge watching it on netflix and giggling to herself and maybe even confessed to a few people like her mom or friends about "your plans" this might cause her to feel humiliated to her parents or coworkers and the time limit might be to save face and not because she is demanding.

So what you can do is give her some time to mourn the loss of her fantasy and straighten things out just like if she has had a miscarriage of a child. Yes I know that sounds extreme but its coming from a place with a lot of emotion and societal expectations. She also sounds a bit younger than you.

You dont need to propose but what you do need to do is reassure her that you love her and are commited to having a longterm relationship with her. That you do see a future with her and that you would be interested in livivg together and getting married and dare I say having a family with her at some point. Say all that and then shift the topic to Christmas. Suggest doing some things like starting a few xmas traditions like decorating together or plan to spend some time with eachothers families to show you are investing it her longterm.

Because what you wrote is "I told her I never planned to propose to her" amd the word never there sounds like you are using her for sex like a pump and dump while in school before you ditch her for Barbie.