Overwhelmed by jealousy

I never experienced jealousy in a relationship. But recently I've been going to therapy to deal with some childhood shit and I'm a mess.

My girlfriend of 3 years and I have recently started seeing this girl. It's all pretty new, only a few months. We communicate very well and it's beautiful. But there is this guy that she was already talking about before we were together and last weekend she finally made a move. She let us know in advance that she would and she is just over the moon with how it went, how she could finally bravely open up and how he reacted and how they had sex several times, etc. I want to be so proud of her as opening up isn't easy for her and it's only thanks to being with us that she's starting to do it. I want to be so happy for her with how it went down. But I'm consumed by jealousy. It's really overwhelming. I feel angry, disgusted, scared as fuck, sad, guilty, insecure. I'm not even able to support those feelings rationally as they are all over me.

It doesn't help that I've just opened up this abyss in therapy that their weekend just keeps throwing me in. But it's really hard to talk to her. I'm also feeling jealous of my long time gf flirting with colleagues, something I've never experienced before even though she was always flirty. it feels like I can't go through with it, I can't love anymore.

I'm well aware that my therapy really messed me up, for now, that I'm healing an old pain and it's just pouring out into the present. I just need to ride it out. But it's really hard to cope, to find the will to love, to live, to live together.

Alright thanks for listening and supporting me in my hard times.