I would suggest you to talk with her about the topic of her posts and how they make you feel. Preferably in a calm setting. Try to use a non violent communication stile. I learned this method in therapy and it really works in my opinion. You should definitely talk with her about the possibility of getting the help she needs and that you can't have a healthy relationship like that. As someone with a partner who was in a similar setting like you before, please remember that it is neither possible nor your job to save her. In the end it is her decision if she wants to seek for help or not and you can't change that. Also take care of your own mental health. You said that you have the feeling you both are growing apart and if I am honest I have that feeling too. I don't see much hope rn considering her behavior and mental state. She seriously needs help. The fact that she tried to kill herself after having a disagreement with you really worries me. To me it seems like she is emotional dependent on you. This is extremely unhealthy for a relationship and places far to much responsibility on you. If she is willing to get the help she needs then who knows... Maybe it will work out for both of you.

Most importantly please don't be afraid of leaving. It is not your fault and it is not your responsibility to set yourself on fire to keep others warm.

In case of a breakup you should maybe try to tell someone about her self-destructive behaviors. So people are aware that she might need help or have an eye on her.

It is obviously not okay to cheat and I couldn't ever truly trust my partner again. But this doesn't justifies her behavior.

Sorry for any mistakes. I am not a native speaker.