Driving larger vehicles is definitely mental(all driving is but in larger vehicles it's even more so). You may notice that you become mentally and emotionally tired while physically you feel okay, that's normal. It shows you're using that gray matter upstairs.
Night time driving is not much different and if you can, you should work on that and build up confidence in that as well. Nothing is more annoying than trying to sleep somewhere and have lot lizards or the law come a knocking. That's why most try to hit up Walparks and truck stops but you can become fatigued anywhere making the first place you can park your safest and best option. There were a few times where I ended up on a highway entrance(NEVER use an exit ramp!) ramp for a few zzzz's before continuing on.
You have some time left before the "big day". Is there a mall or big parking lot somewhere nearby where you can drive around and get used to your ride? This way you can get used to the sounds it makes while moving and turning, you can practice driving in lanes, parking in between lines and get used to visually lining yourself up via the mirrors.
Once you are more comfortable with that you can jump on the highway, go up a few exits then turn around and come back, taking longer trips each time. All of this will help you spatially and assist you in understanding your turning radius. In addition to building your confidence you'll also be building up some road stamina which will make your "real" trip much less stressful and much more enjoyable.
Lastly, "fear" itself can be a good thing. It assists us in keeping focused, aware and attentive. What's not good is when intense fear and/or paranoia steps in. If that happens pull off at the first safe place you can, work it down using whatever relaxation techniques are comfortable for you until the intense fear eases, it will pass. Take a few deep breathes and continue on your journey.
When I get my bus I'll be doing the above as well. Sure, I drove big rigs for 15 years but buses are much different and I'll have a difficult time retraining myself, especially making turns without a pivot point at the 12 ft. mark. I am not spatially aware regarding buses with the exception of being centered in my lane. I have a lot to learn and some confidence to build up as well.
I'll almost guarantee you that once you make your trip(and barring anything additional such as mechanical failure) you'll be looking at your bus and telling yourself that wasn't so bad.
