I mean, knowing that WP would be at least giving it a go with AP except AP ended it/disqualified herself, I don't think a BP should ever get past that. That BP was second choice and was lied to all this time....It's extremely real in every sense of the word. I don't think it is really relevant that WP was in lala-land.
That looks like the crux of the OP.
I equate 'get past' with 'heal'. And I equate 'should' with 'some asshole is telling me I have to _____'. And I equate 'the A was fantasy' with 'I would consider a fake relationship to be one where inauthenticity is so significant that there really is virtually nothing there when you take the fakeness away' (from Stillconfused2022).
I recommend taking 'should' out of everyone's thinking. You want to heal? Then heal. You don't want to heal? Don't heal. But don't screw yourself up with 'shoulds' that you put into in your own head. (You had help, of course, but in the end, you choose to accept the shoulds and you can make a different choice today.
And I recommend not locking yourself up by ignoring that words are metaphors, and many words have multiple meanings. Of course the A relationship exists; it's real in that sense.
But aps are generally in the As to deny their realities. They're telling myself things like,
I'm not hurting anybody
Nobody will ever find out
I'm OK because my ap loves me ... or do they?
This relationship is so special, so fated by the universe/God/twin flames/fate/love/etc/, etc/, etc.
... other untruths that we all can list....
I'd call As unreal because they're rooted in lies. You don't want to call them 'unreal'? Fine. Just don't deny that they're rooted in untruths.
The one you love leaves for an ap? Now THAT gives you revenge without needing to take any action yourself.
If the ap ends the A, that could mean a number of things. Your WS may, in fact, consider BS to be a boobie prize. OTOH, the WS may always realize they had seriously fucked up and at a gut level the BS is what they really want. And that's just 2 of many possibilities.
If the WS leaves, the BS HAS lost the ability to decide, but the BS still has decisions to make - what does the BS want, given the new sitch? What is the BS willing to do to get what they want, given the new sitch?
In many respects, though, the BS has the same power and the same necessity as they have in any failure scenario. They have to choose between letting the failure define them for some part of their future and picking themself up, learning from the failure, and putting it behind them.
*****
Many propositions - like 'the A relationship is unreal' - can be used both as an escape from reality or as a way of getting closer to reality (and in other ways, too).
I have no doubt that the fantasy aspects of an A are used as part of denial. Very soon after d-day, my W told me that both she and ow told each other they would never leave their Hes. I was initially elated ... until I realized W had left me every time she thought of ow, talked to ow on the phone, tested or read a text from ow, had to take a nap because she lost sleep over her discomfort with her A, actually left the apartment to be with ow .... She actually did leave me to focus on ow. IOW, what started as elation because she wouldn't leave me pretty quickly turned into, 'She DID leave me.'
So 'fantasy' can help during the denial phase, and it can help during other phases of recovery, if it is used as a healing metaphor rather than a denial metaphor.
But here's the thing, part 1: I found it natural to see the relationship as fake. I found it natural to see my W's A as a symptom of 'heart-sickness'. That was basic to my wanting R. I strongly recommend that you don't try to tell me I was wrong - I was here with her; you weren't.
Here's part 2: Other people see the real aspects of the A as much bigger than I do; to me the fantasy is paramount. Some people see the cheating as primarily an attack on them and/or their M. I'll insert myself to let those folks know they have choices, and I may give them feedback on what I think they're doing, but I'm not going to tell them what or how to think, and Im definitely not going to tell them what they should think or are thinking.
IOW, we can read a post and decide it we think the poster is in or out of touch with reality, and it makes sense to provide feedbak, but none of us knows if another poster is denying or accepting reality.
******
If you're infuriated by the proposition that 'As aren't real', I recommend looking at why you're furious over an idea that has a base in reality. Does the fury help you comprehend something in your sitch? Does it prevent you from comprehending something in your sitch? How does the fury help you recover/heal from being betrayed? What does the fury do for you?
I can almost guarantee this: if strong feelings are associated with an idea, one will benefit from figuring out the triggers.
And I can almost guarantee this: one is much more likely to identify and implement good solutions when one isn't losing energy to triggers.
That's JMO, but I've got a lot of experience.