Disappointed...
You're probably falling down on essay writing technique/exam technique, especially if you haven't been in education for a while.
In my uni, to get a good grade the writing style needs to be clear, well laid out (lots of signposting) and importantly - to get above 70, needs to be critical, with a use of a wide range of evidence.
I'd suggest meeting up with one of the markers to go through one of your last assignments to see if you can improve your style.
I agree that technique is probably one area where you could use some advice from people in your department. There is a very particular way of writing academic prose - quite dense, with tight structure and focus. Worth getting detailed advice within your department if there's someone who can give it to you.
I'd also say that yes, if you want a distinction you need to read more. It's difficult when you're working, so the most important thing is being focused in the reading, picking things that give the best value for the time. That may be focusing mostly on the assignments, but it is worth following other areas that interest you too, when you have the time - I found that some of my highest marks came from making connections between ideas from different areas, rather than sticking rigidly to the latest things I was coming across. And that made me a better all-round student (and now researcher) too.
But also, don't beat yourself up about marks! My marks were much lower at the start of my Masters than they were at the end - I've heard it described as 'exit veolocity', where you're getting better at your subject as you get further into it (and closer to the end). If you're coming back to studying after a long break, I think this phenomenon tends to be even stronger!
