NightJay, I know exactly what you mean. It used to happen to me too.
It took me a long time to figure what the problem is and its this: you are stuck at a non-existant level of detail. When you are thinking about a mission, only certain levels of detail can actually exist in any meaningful way.
For example, "I want some infantry to attack Goisse" is a meaningful level of detail: it is something which you could carry out. "I want to put a sniper in that bush" is a meaningful level of detail. However, "I want to put some machine gunners defending the town from attack from the west" is NOT a meaningful level of detail.
Why not? It looks perfectly reasonable on the page or in your head. What's the difference between it and the first or second examples?
Well the second is easy to achieve - that is very precise. The first is also fairly easy to achieve - it is very vague. But the third is somewhere in between. It begs too many questions. Where are the gunners going to go - in which bushes? Are there even bushes for them to go in? What are the fields of fire like from those bushes, do they make any sense? What about their flanks? And so on.
What's happening is that there is a flaw in your mission design technique. You are thinking "I'm going to make a mission where some infantry attack Goisse" which is a perfectly sound idea. You then lie in bed and think "I'll have some machine gunners defending the west, and a BMP defending the south, and a couple of snipers defending the north, and leave the east a bit vulnerable." And then you think "Well where are the machine gunners going to go? Well I'll have to look at the map first and see".
The correct technique is to lie in bed thinking about the circumstances and forces available to the local commander. "Well the defenders have only recently taken the town so they are a front line unit: lots of good weapons and highly skilled soldiers, but probably a bit understrength because of casualties. Not much in the way of tents, but probably a few sandbags.
Then, in the morning, you open the map and Preview the mission, with a player unit and nothing else. Walk around Goisse imagining that you are the local commander. You know roughly what forces you will have (because you decided that in bed last night) so you can say "right, machine gunner in that bush. Line of sandbags there with 2 soldiers behind it" and so on.
In other words, do the design in your head that can best be done there (story, background, circumstances, etc) and design on the ground the things that can best be done there (unit placement, angles of attack and so on.)
Hope that helps.
PS - two specific tips. Firstly, never stare at the Mission Editor screen trying to decide what to do. Preview the mission and get on to the ground. Secondly, when you place a unit, always make it playable. Preview the mission and test the spot, then think "where would I rather be?" and run around a bit. The Mission Editor screen is for creating units, and moving them to the right place once you've decided where that is. You can't tell where a unit should be just by looking at the map.