I noticed that I'm not good with mid/late games. I win easily if I rush the enemy. However, I like to build up when I'm in the lead, but I am not good at it.
Chances are, I spend a lot of time treating the game as city builder and when AI rushes, my army are too spread out. And when I'm rushing, I can't keep up with the reinforcements, so when my wave was blocked, I'm bascially sitting ducks as AI would slowly encroaching my base.

I understand, however, I noticed that during mid-game I tends to treat the game as city-builder and be more relaxing. Thus, "no unit that does nothing" and "no building produces nothing" doesnt seem fun to me, and the reason why I'm being overwhelmed by the AI.