I discovered my girlfriend’s alternate social profile where she posts complaints about me

My girlfriend and I have a great relationship, or at least that's what I thought until yesterday. She left her tablet unlocked on the couch while she went to walk the dog. A notification popped up from an alternative Reddit account she created, and curiosity got the better of me.

I opened her post history, and I’m completely heartbroken. For the past year, she has been running a secret account dedicated entirely to posting detailed complaints about our relationship on relationship advice boards.

She has posted about our private arguments, criticized my career path, complained about my family, and even asked strangers if she should break up with me because she feels "bored."

In person, she is always incredibly sweet, supportive, and tells me she loves our life together. Reading her true, raw thoughts written for thousands of strangers online feels like a massive stab in the back. When she came home, I didn't say anything because I don't know how to admit I looked at her tablet, but looking at her now feels completely fake. How do I bring this up?