For variety try changing up the preferences or having a drasticly unbalanced force. (i.e. 30 tanks and 1 platoon of infantry against a ton of anti-tank guns.)
I know when I get discouraged I will whip up a generated battle that is likely to be a walk in the park where I can shoot pigions all day. Don't get hung up on the metaphore, Just set up a game were you will be able to win without too much trouble. It always get's my moral up. Hit the "max points" button, buy lots of the good stuff, move the objectives and change the map so you can set up the "perfect" defence (or attack, or whatever you want to do.) Have a blast at the AI's expense.
Everybody needs a moral boast every now and then.