Chrissy Teigen is owning up to her past abusive behavior toward Courtney Stodden -- publicly apologizing for years of online bullying, and promising to be better.

Chrissy, who's back on Twitter again, issued a heartfelt mea culpa on the platform Wednesday ... this, after Courtney revealed Chrissy used to DM back in the day telling Courtney to kill themselves, and that was on top of her ugly public tweets.

Chrissy now says ... "Not a lot of people are lucky enough to be held accountable for all their past bulls**t in front of the entire world. I’m mortified and sad at who I used to be. I was an insecure, attention seeking troll. I am ashamed and completely embarrassed at my behavior."

She adds that as bad as she feels now, it's nothing compared to how she must've made Courtney feel all those years.

Chrissy goes on to explain that she's tried connecting with Courtney privately, but to no avail ... so she says, "I’m so sorry, Courtney. I hope you can heal now knowing how deeply sorry I am."

It's interesting, CT ends her apology by also saying sorry to her fans -- for letting them down -- so she's covering all bases here.

You'll recall, Courtney told us they considered Chrissy a hypocrite when she took a break from Twitter because of all the negativity ... saying that didn't seem to bother her all those years ago when she was jumping down Courtney's digital throat.

Chrissy didn't respond back then, but it looks like Courtney's latest reminder of the abuse was one too many to ignore.