I actually like this climax. Things are coming to ahead and emotions are exploding out. Let's go! It's also an interesting twist when the main pair is already dating. In real life standard case it's completely understandable for one's significant other to be uncomfortable with their partner spending time with someone of the opposite or same sex (someone that could be a romantic partner) alone and in this case the FL knows the other party is actively trying to pursue the MC romantically. It kinda goes back to her own insecurities. She doesn't need an excuse to stop a literal homewrecker from homewrecker, but also the excuse is effective. She's being more gung ho in their relationship while giving herself pretense.
I agree with the general consensus that the other woman should've, would've, and could've. Since you were a child isn't something to be proud of. It only means you were either too scared to be honest with your feelings for that long or too complacent thinking he'd never get into a serious relationship with anyone else. Regardless it's not an excuse to try and overturn his life now and center everything around yourself. I mean at this point you've just attacked his actual girlfriend for actually genuinely helping him edit and produce a better quality video (which is kinda funny and some serious shade from the female lead; kudos).