I'm really sorry to hear about the predicament you've found yourself in, looking for accommodation is always super stressful!
What's helped me in the past is joining Facebook groups - such as those titled "Cardiff Freshers" or "Cardiff Accomodation" or "Cardiff Housing". There's usually an abundance of students offering rooms on these groups. There are always other students who offer their place to others for the year for certain circumstances, for example dropping out, not getting their offer, or deciding to switch to a placement year.
I hope that has helped!
- Fatiha, Cardiff Uni Student Rep