WST.
Don't stress, your experience is normal - especially one with only 400 hours flying a big shiny jet. Everything seems to happen at light speed compared to a Seneca !! In a big jet, you must make corrections early and not let the aircraft deviate from the flight path without correcting it, otherwise, it can very quickly get out of hand.
The way the human brain learns something new is that, initially, it uses it's entire conscious mind to think about and supply answers for the new experience. This is why you may feel that you have very limited capacity at the moment. However, after a while, your brain will learn to do a lot of the donkey work automatically, without you thinking about it.
I would warn against overloading yourself by trying to learn everything about the route. Just find out whether there is anything unusual about your departure and destination; if ATC tend to keep you high or fast on approach for example - and if so, how to deal with it. Make sure you know your company's SOP's, your cockpit set-up drills, (after engine start, after landing etc.), how to operate the aircraft, and how to calculate a descent point. Also, try to know the flight paperwork that you as F/O will have to fill out. A trainer will not expect you to know everything, but if you can come across as generally competant, and are not having to be 'spoon fed' about absolutely everything, you should be OK.
If you don't recognise the full name of a VOR, you can ask ATC what the 3 letter code is - no shame in that - I do it and I've got 7,000 hours !! Usually, though if you look at the PLOG/Flight plan, you will be able to spot the three letter code they mean, and if not 100% sure you can just confirm it with your trainer - "KRH is that?".
If you were doing a driving test, your examiner would not expect you to be able to navigate a route from memory, but they will want to see that you can operate the car safely - it's the same thing with aircraft.
Good luck !
U