The teen, who is a UK size 6, has also posted about airbrushing her skin, editing her "uneven" teeth and feeling "uncomfortable" about her nose.
In one post, she claimed a modelling agency told her that her hips were "way too wide" and that she needed to lose weight "in order to have a chance at London Fashion Week".
She explained: "I already was underweight so I decided not to lose any weight."
On a black and white selfie posted on Tuesday, she wrote the caption: "At this point the only reason I'm still on Instagram is [because] casting directors started asking how many followers I have so they can decide whether they consider me or nah."
She added: "[I don't know] how to feel about this but they this is me dropping a selfie to stay in the game."
In another post, she talks about cutting out salt, sugar, fat and alcohol "for months" in the lead-up to London Fashion Week to keep her skin clear.
Jazz also admitted to deliberating for over an hour before posting a photo on Instagram because she felt "uncomfortable" about her nose.
In a full-length mirror shot, she wrote: '"This took me like 15min to take and I was close to falling off my bed [because] I had to make my legs look slim."
Her followers have praised her no-holds-barred attitude and thanked her for her honesty.
Jazz told FEMAIL: "Beauty standards are hella stupid. And what's going on behind the scenes of the industry is even crazier.
"For example, most designers only create fashion pieces in one size; usually us size 0-2 (UK 4-6). So when they go on the hunt for fashion show models, they look for girls who have a 0-2 size.
"That's one big issue. I think, if more designers created standard sizes, the pressure to be underweight would decrease. But that's one issue in a million. There are tons of reasons why the industry needs a revolution. Promoting a healthy body image is so important."
Speaking about her Instagram posts, Jazz said: "I think, even if only one single person thinks about my words and changes their mind when it comes to the fashion and media industry, I already feel like I'm doing the right thing."
She added: "I get so many messages from young people who ask me how they can lose weight or other beauty questions.
"I never give them advice, I only ask them: 'If you were the only person on this planet and no one could see you. Would you still want to change your body?' I guess you know what they then answer."
