Can you not delay Met to the next session? The exam will still be there when you will feel more confident to sit it.
Also I hear you on OAA books, I have given an extensive look at them and they are certainly nice to have, but they wouldn't be my primary source of studying. I suggest you get EASA Professional Pilot Studies in PDF and study Meteorology from there and then watch OAA's CBT – I am sure everything will fall into place as this is my studying strategy. It works for me, it might work for you too.