I don't know how it works where you are, but over here if you fail one part of a test you get a re-test on just the failed section. Anyway, I failed both the initial CPL test and the partial re-test. It was the same story as you - I knew everything but got myself in a spin (not literally!) under the pressure of test conditions.

One thing that helped me was a mental reframing of the situation. You're clearly aware that your knowledge is there, but right now you're probably hoping that external factors will allow you to put that knowledge to use - good weather, a nice examiner, a quiet traffic pattern, whatever it might be. I say this because it's exactly what I'd been doing. I realised that it wasn't actually the test itself I found stressful, it was the feeling that the outcome was out of my control - something external might come up and ruin it.

The key thing is to realise that the only one who controls the outcome of the test is you. And that's incredibly liberating. You already know that you can fly well enough to pass the test, otherwise your instructor wouldn't have recommended you. So if you want to pass, turn up and pass. The external factors have no power over you. Bit windy? You did crosswind landings in training. Nasty examiner? Fly to standard and he can't fail you. Busy airspace? I'm sure you've dealt with that before. Since this realisation, I've passed both an IR and a type rating first time.

I understand your post is probably more of an in-the-moment vent rather than a genuine wish to give up, but you should think on it before doing anything like that. I had the same feelings when I failed an interview with one of the few companies willing to overlook a CPL failure. You passed the re-test, and you said in another comment that you already have a PPL, so clearly you can pass tests. But you need to find some way of mentally handling the pressure because an airline will test you every six months.