DID in young adult/contemporary fiction and how to portray it?

Okay, I'm planning a book, a love story, about a girl with DID (AKA multiple personality disorder) in high school falling in love.
How can I portray multiple personalities realistically?
Any advice would help!

To be properly realistic you would need to:

(1) acknowledge the rarity of DID in real life compared to its incidence in fiction;
(2) find a reasonable "reason" for the MC having this condition - most cases appear to result from severe abuse as a child;
(3) recognise the high probability ot your MC also suffering from another mental illness such as severe depression, bipolar disorder, PTSD, anxiety, or even psychosis;
(4) note that sufferers of DID are also far more likely to develop substance addictions or eating disorders.

In all seriousness, if you can tackle this subject sensitively and realistically in a YA novel then you should have something really special on your hands. It's not easy stuff to do!

Moving to YA & Children's Lit

If you're planning to make this "realistic," you've got one major problem already. If the character has actually got a full-blown mental illness (and she's a bit young for that, although it's not impossible), she doesn't belong in a regular high school. People like that can be dangerous to themselves or others, and they need special care and supervision.

of course, that doesn't mean her parents aren't sending her to regular school anyway. But they'd have to be truly neglectful, as would the school staff, to not notice a severe mental disorder in one of their students.

And dissociative identities, if it exists, is severe. It is NOT like mood swings, trying on a new subculture every month, gender questioning, pretending to be different people, or even compulsive lying -- those are fairly normal behaviors for teens. This is like multiple people all living in the same body, and nobody's reliably in charge, and most of them have disorders of their own. Bad news for any kid that's around her, or any adult that isn't trained to cope with mental cases. A person like that is capable of anything.

People with this disorder are able (from what I've read and for the most part) to live healthy and fulfilled lives, in spite of or because of their alters.
Why is it so ridiculous to have her attend normal schooling?

You'd first need to work out a plausible backstory. Most DID patients report serious childhood abuse or trauma, although there is enormous difference of opinion about what causes or triggers it, and...well, everything about it, pretty much.

It's a controversial disorder, and I don't know enough to give good advice about portraying it. I did once meet someone who was in treatment for it. It was a bit frightening. In order to do it realistically, I'd look for some video or other accounts online by people being treated for it, and look for common details. One question to ask yourself is whether she and others have identified what's going on yet. If they have, she will probably not be in mainstream school. She'll be getting a lot of psychiatric support. If they don't know what's going on, that's also extremely complex.

The biggest issues I can see for a young girl falling in love, especially if she's not aware of it, is that first, people with DID tend to have massive trust issues, and secondly, her alters may not be on the same page about romance. Some of her may be distrustful, resentful of the intrusion the boyfriend/girlfriend represents, terrified of romance or sexuality...the probability is that not everyone inside this girl's head will be comfortable and cooperative with a romance.

If her condition is known, people will tell her she can't be romantically involved. She may not listen--what teen does? but she'll encounter resistance from a lot of people.

It's enormously stigmatized. You're going to want to examine how her partner, if aware, sees this, and how the adults around them do.