Napoleon once said "no plan survives contact with the enemy." You'll get some unlucky battles, you'll get some unlucky ones, and some of your enemies will do unexpected things. If your plan requires everything go right for you to succeed -- well, everything never goes right.

For every turn, develop a plan. Have clear objectives, and know which are the most and least important. Dedicate the most armies to the most important objectives, even if that leaves too few armies for the less important objectives -- it's ok if you can't take them, they're less important.

Make sure you have a small number of objectives, and develop simple plans to gain them, because simple plans have fewer components and are less likely to get disrupted by your opponents' moves or by your unluckiness.