I have this problem too - and what's worse is that in Revs, you'll get one 10-mark question that's a page long, and it asks you to discuss events that occurred in the space of a year - very, very, very broad question to write about in one page/15 minutes. What I do for essays is plan EXACTLY what I'll write in each paragraph, and then also write down how many words it'll take me to talk about that. Check your word limit after each paragraph - it's probably easier to cut down, paragraph by paragraph, than to look at a completed essay that's 1000 words over the limit and think 'well, what the hell do I do now?'
Part of the reason my history responses were so long at first was because I had a bad habit of 'telling the story.' Assume the reader is at least a little familiar with the narrative, and don't waste all your words on describing the events - instead concentrate on the argument you're trying to make. Instead of talking about what exactly happened during a particular event, discuss the significance of that event - that sort of thing. I used to write a whole paragraph about what occurred, and then another whole paragraph about the effects of that occurrence, but lately I've just concentrated on the significance of the event rather than the details of the event itself.
Also, don't be too concerned about including everything that you've learnt. I always like to cram my essays full of stuff, in an attempt to demonstrate that I've remembered absolutely everything that has ever been mentioned in class, but it's better to choose a few key points and discuss them in detail.