My gf broke up with me and it’s breaking me

My gf broke up with me about a month ago, pretty abruptly under the excuse that she needed to find out who she was on her own without anyone to guide her. I am pretty inexperienced in dating, this was my first longer relationship lasting about 16ish months. At first the breakup hit me hard because I was already battling mental health, but I took it pretty well, distracted myself, and got out of negative thoughts kind of well. I sort of kept in contact with her, and eventually it wound down to me unadding her on everything so I wouldn’t remind myself of anything bad.

Fast forward to today, sometimes things remind me of her so I check up on one of her accounts to see if she’s still living her best life. Bad idea. She posted herself with this other guy and he’s kissing her. It hasn’t even been a month since we broke up and she’s moved on and is already with someone else. It feels terrible to me because when she was breaking up with me she felt bad about hurting me because feelings are feelings and it wasn’t really my fault that we were breaking up, but now it all just feels like a lie and I don’t know what to think anymore. I feel like all of that slow buildup of resistance to the negative thoughts I had about the breakup has just shattered and I’m scared I’m going to go into a really negative spiral over the next few weeks. It hurts.

Tl;dr Dated girl for very long time, she broke up with me because she couldn’t grow while in a relationship, and is dating someone else not even a month later. Watching it all from the sidelines is destroying me.