RE: "The end of the world is here and I must choose between my boyfriend and my Ex" – Using our dreams to make decisions.

I also think the answer of why they invite their ex in their Dream can only answered by them, I loved to read this paragraph of yours,
In the morning you may remember a dream in which you clearly knew what you want and what to do, much like the dreamer above. If nothing comes up, breathe in deeply, and tell yourself that the answer will come to you during the day. Release the dilemma from your thoughts as much as possible (this is important!), and Trust yourself that the answer will come
It's simple but contains a valuable information.