Actually, what I see is the AI doing the smart thing by attacking your weak spots. The trick is to eliminate these weak spots, OR take advantage of this knowledge and move your large force to the weak province and meet his force on your own terms.
You can use province defense (PD) to beef up a province and make it less appealing. It is also wise to back this up with a fire team of mages, bowmen or whatever depending on the type of enemy you are facing. Also, look for choke points and plan your defense as you advance through independent provinces. Use strong independent provinces to your advantage. Leave them where it is convenient to create an additional barrier. Use fortresses at strategic locations to slow any runaway breaks in the line.
Hope this helps.