When she first learns of her fate, Brittany rampages. When you have a destiny like hers, you're chosen - you don't get to choose. She rampages because she's angry and feeling helpless. Why does it have to be her? It could have been any one of a million other people, but it's going to be her. And she's just a girl, after all. Like any girl, she's frightened at the prospect of leaving her family and the Tubbingtons and Sam, and she doesn't quite understand what MIT will be able to do for her. The way that the professors had looked at her was very odd, like she was a interesting piece of sterile lab data, instead of a beautiful unicorn with a rainbowy horn. She hated the thought of moving to a strange city, dancing in strange streets and strange classrooms. It's frightening to think of mixing with the other students, who'll look at her like she's a freak, and laugh. Sam had never laughed at her, and neither had Santana, but that's because they both had understood, and Brittany realizes too late how rare the gift of understanding really is.
So for these problems, Brittany doesn't have any easy answers. If only they were as simple as the others. She'd rather have a problem like Ryder's (she knows the answer: just love her), or Kitty's (she knows the answer: just love others), or Tina's (she knows the answer: just love yourself). Those solutions are exquisitely simple and she wishes she could just sit them down and explain, but what she knows, deep within the marrow of her bones, doesn't come out in a way that makes sense to them. That's why people dismiss her. She walks among mortals and speaks in the tongues of angels and the discordance between these worlds means that Brittany is often left to walk alone on the convoluted paths between heaven and hell. She moves swiftly, like she dances, with celeritas: in a blur, as fast as the speed of light. Other people can't catch up to her to take a better look, and so they do not understand.
And as for herself? She'll figure out eventually what the plan is. But for now, she does what she can to relieve her frustration. Her Cheerios uniform goes up in flames, and she dances wildly inside with joy, because fire cleanses. The Glee kids, and Mr. Schue, and Sue, look at her askance when she cleanses them too, but she won't be here anymore. Actually, it's not that she doesn't care about them; it's that she'll no longer be around to care for them or to reveal the truths that she can access so easily.
She learns to accept what is coming. It's hard. Sam and Santana help her. Her other friends are sad, and of course she is sad, too. They all weep. It is a parting on many levels, because they'll never meet on the same terms again. They will stay grounded, happy, secure, and she's off like Icarus, off to flirt with the sun, praying desperately that she won't fall. They're her friends, and they took her in and supported her when the rest of the world would not. She tries to comfort them, but again, she has that problem of communication. And while they do know she is sad to leave them, she can't quite say what it is that she needs to say to them.
But if Brittany could speak in ways that they could understand, she would say this: that, as Kurt and Blaine know, there are no such things as forever goodbyes. Light talks to her and this is what it says: matter interchanges with light, light is everywhere, energy is everywhere, and it's constant; energy cannot be created or destroyed, but it is transformed. She's going to transform, too. Her energy will always be somewhere in the universe, even if it isn't in Lima, or New York, or wherever else the other Glee kids end up. Her heat will continue to steam off of the things she's touched. It's just that it'll be a little more disordered without her, but she'll still be there. She understands entropy and its connection to energy. That was another reason why the world of order and mundane things never understood her, either.
Brittany doesn't know the textbook definition of the word photon, but she does understand that as light strikes off of you, you change its trajectory forever as it whirls forever, never stopping. She hopes they understand how exactly they relate to each other better, because their encounters change each other's fates, just as bouncing, colliding photons change each other's fates, too. So she hopes Ryder and Unique will figure out what lies between them, and that Kurt and Blaine will figure out what lies between them, and that all the others will figure out the web of light that lies between all of them. She wants them to move with celeritas, because you don't understand light and goodness and energy unless you do.
She does what she can to tidy up matters before she leaves, and then it's time to go to this strange new place called MIT. It's unfortunate that the Glee kids will have to figure all of this out on their own. She'll always wish them well as she wings her way towards other worlds, but she's done all she can. She's going to embrace her destiny, and the light that talks to her.