So been dating this girl on and off for around 2 years now.

She's great in many ways, smart, fun, etc.

But the problem with her is two fold: she's highly emotive, and requires constant validation.

Emotiveness:
Her life is run by her feelings, not reality. Kind of a problem since reality happens regardless of her feelings. Not the critical issue by any means but kinda annoying. Every girl in the world is kinda like this, but she's quite over the top about it in that, for example, her organisational skills are horrendous due to the fact that she'd never planning ahead and only doing what she feels like.

Validation:
She needs constant validation. Good or bad, she's always turning to me for me to give her feedback, be it reassurance, empathy, or whatever. I think it stems from her family being so nice to her her entire life. A mutual friend once said "her family is the kind of place where even if you came rock bottom last in a race, the parents would congratulate you on how well you did". It breeds emotional dependence. Combined with some low self esteem, it's not good. (she's HB9, but still has massive confidence problems).

So the emotiveness element is tolerable. Most girls are like this, and she's just a little emotional most of the time. But the validation part is really starting to bug me. At first it was pretty mild but after being with her for this long it feels like the only thing she talks to me about is to seek validation. If she's sad she wants me to reassure her that everything is ok. If she's feeling low with her self esteem she wants me to tell her that she's awesome. If she reaches some goal, however large or small, I need to validate it by congratulating her.

It's really starting to drain me and recently I've been completely indifferent to her behavior. I was quite in love with her but recently it feels like I've become pretty tired of it all. I'm a firm believer that you shouldn't be in a relationship trying to change someone, and i'm not trying to do this, but maybe there's something I can do to better the situation. Thoughts guys?

A girl with self esteem issues is not a good choice for a LTR in the long run. However, her age is a factor.
But do bear in mind that from low self esteem stems cheating most of times.
As you know, all sluts out there seek validation through sex due to self esteem issues.
You answered your own question here, go with that and break up with her, you are your main priority.

These kind of girls go one of two ways: they either obsessively seek your reassurance or they find the attention elsewhere. The problem is you've allowed it to go on for 2x years and she's now become accustomed to you being passive and submissive. It's like the teacher at school who was really cool / relaxed, then suddenly being really strict. You wouldn't respect him as much as the one who was strict from the start.

You can try giving her less attention. She may seek it elsewhere though. I usually give these kind of girls lots of space in a relationship, so that I don't become their only source of affection. If they are getting reassurance from friends and family, it takes the pressure off you. Give her space.

^This, but in the meantime make sure you bang her like noone else, to make sure she's not seeking dicks elsewhere besides validation.

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I've always given her a lot of space and handled her requests for reassurance like a shit test. It's just that now she's really enforcing the issue like asking me why i'm not reassuring her, and getting upset about it, instead of just moving on.

Also, no way she'd ever cheat on me. Nothing to worry about there. She might lose interest but it's almost like I wouldn't care anymore.

Cover your nose Matty! She's doubling up on the shit tests.
If she's your long term girlfriend then you should just sit her down and tell her to chill out. If she respects you then she'll give you the space you're asking for. If not then you're fucked.

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The ultimate lesson of psychoanalysis is that human life is never "just life": we are possessed by the strange drive to enjoy life in excess, attached to a surplus which derails the ordinary run of things.
-Slavoj Zizek

Never, ever say she definitely wouldn't cheat on you.

You would be amazed my friend.

I was once in a relationship for about a year and a half. Towards the end of the relationship I started seeing behavior like this on her part. She has lost respect for you/forgotten what you mean to her/needs to appreciate you more. If this kind of attitude is constantly present I would say it is one of these or a combination of them.

She should be trying to please you as her boyfriend like she probably did when you two got together in the early stages. My guess is she doesn't really have anyone else to confide in and so she is just venting to you because you are there. Some venting is good and healthy, but don't take too much of it, and remind her that she is lucky to have you as a boyfriend.

I remember my girlfriend once came into my apartment drunk and crying about how she hated her life.... I realized then and there she had actually lost respect for me and I refused to be in her company. It's like a child crying. You want to help but there is nothing you can do sometimes. Or at least that's how my experiences relate to what I read in your post.