Well sometimes you have to admit to yourself that the huge scenarios are not for you!
It is very common for players to start a large scenario full of excitement and anticipation only to find that they have bitten off more than they can chew and the game inevitably fizzles out, I found the best way to handle large scenarios was to build up to them by playing the smaller scenarios and then slightly bigger and so on, even then I found it a chore at times to play a turn when I had limited free time for gaming, so now I only play the huge scenarios as part of a team which means you are only responsible for a fraction of the units but get the benefit of participation.