Aww man, poor Carrie. It must be horrible being blindsided by a breakup, with someone she clearly wanted to settle down with and then have to contemplate going into work to sing about heartbreak every night!
Potentially unpopular opinion but I don't think the fact that it was sudden and that Carrie is heartbroken means that Oliver must have done something awful. One of my best friends had her ex-fiance break up with her, completely out of the blue, when they were due to look at wedding venues together in a few days. They'd been together for years and she was totally bewildered. Turns out he'd been having doubts for a long time but had suppressed them until he just couldn't.
Carrie and Oliver's relationship is characterised by being the exciting fling (Carrie herself compared what must have been the early stages to her uni experience) followed by a huge chunk of it being spent in lockdown, alone together, with neither of them able to work. They haven't really had a normal relationship for a lot of it.
Perhaps spending so much time together is when Oliver started having doubts about the relationship but then he hoped going back to work and being able to see other people again would make things better. And then maybe they just served as reminders that he was happier away from the relationship than in it. So they go to Lapland and maybe in Oliver's head it's a make or break type of holiday. Carrie thinks it's magical but maybe for Oliver it wasn't.
We've deduced that Oliver probably broke up with Carrie after that live she did, not long after he came out of COVID isolation. She was expecting a quiet evening with her boyfriend but maybe those days of contemplation, alone, are when Oliver finally plucked up the courage to break up with her. Coming out of isolation was perfect because he could actually take his stuff and go.
Now this doesn't mean Oliver is the devil incarnate if it is the case: Carrie could have ignored huge warning signs thinks weren't working. However, it also doesn't mean Carrie doesn't have a right to be hurt or upset: she loved him and she was blindsided whatever happened. Anyone would be hurt.
I might be wrong but I don't buy the comments saying he MUST have cheated/only been with her for her money/be a giant prick. Relationships break down all the time without it being solely one person's fault. I really do think this could be one of those times.