I am halfway through my upgrade to Captain on the A320 (1st command) and I would say concentrate mainly on aircraft limitations, memory items along with FCOM3 approach procedures. After that fill in your 'spare' time trying to brush up your technical knowledge and start thinking through 'what if' scenarios. Pay close attentions to day to day problems on the line during your remaining time as an FO in particular to how minor pax / loading / MEL problems are handled by the Captain. Remember the first thing you should be asking when a problem arises is 'Who else can take this problem off my hands, do I really need to be the one dealing with it?'.
I agree with you, it is a very nervous time during your training, don't take anything for granted, just head down and work hard for a couple of months and your will be right.
Good Luck