BRITFOR here, I'm studying for a Masters (not history) at the moment and I agree it is difficult to fit everything in. My faculty suggests I will need about 16 hours’ work per week. So:
I have the full support of my boss. It helps that the military is sponsoring me, but I keep her informed of my deadlines and progress and she helps me out by being flexible.
I have to work out what is essential and what isn’t. Exam revision comes first because there can be no deadline extension, then essays, then reading and finally online discussions.
I read little and often. I take papers into work with me so I can read a few pages during my tea break or over lunch and I try to set aside 30 minutes each evening just for reading.
It is still difficult though and I think there is as much benefit to be had from learning to manage the workload as there is from learning the subject!