Why am I bad in tactics?

[MoKus]

Thanks for your replies and tips. Here is another battle (iam playing as German): https://ufile.io/tkhpi. Here is the quick save file: https://ufile.io/eb4su. I managed to kill around 390 and lose around 420. Computer more or less score the same. It would be nice of you if you could play the battle an let me know your score. My general has three stars and enemy general has four stars.