AIO to my gf not telling me about having dated her guy friend?

This an 8 month relationship. My gf is close to this apparent guy friend. She initially said he was a childhood friend. I recently found out he’s actually her ex bf of 4 years. She told me their relationship was so bad it led to her being hospitalized for anxiety and got diagnosed with PTSD. I'm struggling to wrap my head around this and feel like I'm blowing things out of proportion.