In the lengthy post, Brooks, 18, claims that Grande, 20, cheated on him with Sykes, also 20, while he was still romancing the Your Truly singer.
"Yes I was cheated on. Yes it does suck. Yes I was left for another man," Brooks wrote. "And this happens everyday to so many people of every age, gender, race. I happen to be one of those people and I'm in a situation where I am constantly reminded of it every day."
The Aussie further added that he was a "miserable wreck" after their breakup, and has just recently gotten to a place where he's "finally ready to move on."
"Through all of this I have been able to mature as a person and grow up and have learnt a lot about myself," he wrote. "I have learnt how to find happiness on my own without relying on anyone else. …Today was supposed to be our one year anniversary but instead I'm writing this. Do you know how sh-t this makes me feel?"
Despite his heartache, however, Brooks insists that he doesn't mean to defame Sykes, only the way in which he swooped in and "interfered" with Brooks' relationship with Grande.
"Nathan may be a top bloke or a sweet person, but what he did was just completely wrong," Brooks wrote. "He interfered with my life and did not respect my relationship, his actions caused me to not be able to sleep at night and not see the world as a beautiful place."
"Nathan if you read this, when you hold her hand you better hold it tight because you are holding my world," he added.
Grande, for her part, also took to Twitter to express her disappointment at Brooks' post, but vowed to keep the high road in addressing the situation.
"Woke up this morning in complete and utter shock …I guess I shouldn't be so surprised," she wrote. "You said to me if I didn't come back to you, you'd make me look bad to the entire world. …I'm no longer afraid of you or your lies anymore. I'm going to be the lady my mother raised and carry myself with class and dignity…But I will never be any man's press opportunity."
"Girls… RESPECT YOURSELVES," she continued. "Keep your head held high and speak only kind words. Know your truth… no one can ever take that away from you. It's always ok to defend yourself. My fans know what kind of person I am. Never let any man discredit u or the values you were raised with. I'm finally happy. Words can't hurt me anymore."by nevel