I have almost finished the training and internship for the PADI DM course.
The thing I have found hardest to get my head round is the degree to which you have to manage both student and newly certified divers. For example, we train in very poor vis (1-2m). If I don't tell my divers to hold the rope all the way to the bottom and keep holding it when they get there, they let go and disappear and I have to go back up and find them. Gngh. Nothing is obvious to new divers, you've got to tell them what to do... but without offending them.
Get used to diving with a lot of extra weight, or not enough. If you carry extra weights, your divers won't need them. If you don't, they will. And when you think you've finally got everyone sorted out, someone will float off to the top.
I spent a lot of time in the pool on my own, practising skills. I also practised buddy breathing on my own (you don't really need a partner, you can just take two breaths and them hold the reg out as if someone else were using it). That helped me to get the rhythm and made the equipment exchange much easier.
At various times I have been utterly fed up with PADI, my instructors, myself... and sometimes all three at once. Be prepared to take whatever the course hands to you and deal with it.
I would like to say it's all worth it in the end, but I haven't got there yet
Good luck!
Zept