Thoughts on not going the distance.

I've read Fluttershy Saves The World in 7 Days and I've got opinions.

First off, I very much enjoyed this story and the outlook on mental health issues, especially pertaining to anxiety (and others that come with it).

Having said that...
Twilight and Fluttershy are silly.

I never really got why people have such problems with outliving others. That's going to happen regardless of immortality/agelessness. In a circle of loved ones, someone will be the last to go. Or often someone will go early. And contrary to what many think, life will go on. It's of course a very romantic and indeed very old idea to say something along the lines of "I can't live without you" but it's far from healthy or even really true. Exceptions are of course cases where the ones left behind may feel guilty (especially in cases of suicide and/or ODing and similar things) or relationships that were unhealthy, to begin with. Those wounds might never heal. But otherwise, the heart will scar but heal, forever marked by the ones we love but working even when they're gone.^{Got a bit poetic there. I like it. And, oh my, I could go on a tangent about self-awareness not being absolution and how neither really matters that much but w/e.}

Thus I feel that calling off a romantic relationship before it can even begin is the wrong thing. Absolutely understandable but still motivated by a fear of failure.

One might argue that this wasn't out of fear but altruism. Because Twilight's important.
1) Twilight is important because of the ones that elevated her. Her friends. With Fluttershy very much among them.
2) How exactly does that matter? Maybe I'm just ignorant but I have yet to see someone (the characters, of course) explain this without defaulting to vague and nebulous non-answers like "It's not meant to be.", "I don't belong in that picture.", "It's for the better/greater good." or similar which really just boils down to "Because I feel like it." Or maybe... "Because I'm uncomfortable."?

It's always a balancing act between "You don't need to if it makes you feel uncomfortable." and "You need to step out of your comfort zone." for both are right and true.

Okay, I'm jumping the gun here, so let me back up a step.

"Because I feel like it" has to be taken without any perceived snippiness but instead at face value. Maybe it should even be prefaced with "I don't know but I guess it's ...". Actually maybe a "don't" would be more appropriate here.

So, to reiterate:
Question: "Why do you not want to pursue this (usually romantic but not exclusively so) relationship?"
Answer: "I don't know, but I guess it's because I don't feel like it." (Again taken at face value)

Now, there are two things here: "don't know" and "feel". I will assume that "don't know" implies a lack of descriptors and not any deeper-reaching mental problems because that would lead me nowhere. As for feel...

Question: "How do you feel?"

Answer: "I don't know. As I've just said." (snippiness allowed this time ;)

Question: "But you said you 'don't feel like it', so what do you feel?"

1) "Uncomfortable"
1a) "I don't want them to mourn me." (in case of immortality add "[...]forever."
1b) "I don't want to grow old while they stay young." (In case of immortality with agelessness)
1c) "I don't want to get hurt/hurt them if it doesn't work out."
1d) ...

2) "Afraid"
2a) "What if it doesn't work out?"
2b) "What if they aren't who I think they are?"
2c) "What happens after I die?"
2d) "What if they don't want me anymore when I grow old?" (Weirdly not exclusive to relationships with immortals.)
2e) ...

3) "It doesn't feel right." aka "Something is wrong." (Further answers here depend entirely on context.)

"3)" brings in a completely new host of issues that won't help in this particular case study (this isn't really a case study).

"1)" and "2)" are closely related and I of course have oversimplified the issue with my earlier statement.

[...]Or maybe... "Because I'm uncomfortable."?

But I wasn't wrong. (IMO but that is kind of the point.)

Now, I could of course try to further simulate this "interview" but considering the number of possible conversational branches and the fact that I'm embodying both sides...

No, I don't think I will.

What this all comes down to is, that I think the reasoning behind Twilight's reluctance to become immortal and Fluttershy's decision to call off the romance is very very silly (so as not to say stupid because I like these characters) and that Angel considers it "Strength" to not try a new route to happiness is actually very unhealthy. One could argue that sticking to your guns is indeed a form of strength but when does that become an unwillingness to change?
As I said before:

It's always a balancing act between "You don't need to if it makes you feel uncomfortable." and "You need to step out of your comfort zone." for both are right and true.

and

Absolutely understandable[...]

But understanding does not beget endorsement.

Does that make their decisions wrong? Not really. It just diverges them from a path I'd like to have seen them on.
This is, after all, just shipping.

Seriously though, the story is great.