Miley just went on a long Twitter rant saying she's accepted a life of being open with everyone ... but she refuses to be labeled a liar and cheater and own up to "a crime I haven't committed."

That "crime," of course, relates to her hooking up with Kaitlynn Carter while still being married to Liam. Cyrus says ... "I refuse to admit that my marriage ended because of cheating."

Miley says, "Once Liam & I reconciled,I meant it, & I was committed. There are NO secrets to uncover here. I’ve learned from every experience in my life. I’m not perfect, I don’t want to be, it’s boring. I’ve grown up in front of you, but the bottom line is, I HAVE GROWN UP.

She adds ... "Liam and I have been together for a decade. I've said it before & it remains true, I love Liam and always will."

Miley's long post also includes a list of mistakes and an admission of cheating ... when she was younger. She lists off several of her most controversial moments, including drug use and swinging on a wrecking ball naked.

But, the gist of it is -- despite what everyone thinks -- her cheating was not the reason for her split with Hemsworth ... and she's grown up and made a decision to leave that life behind.

We broke the story ... Liam just filed for divorce.