First of all, thanks for your positive comment.
I suppose you are playing the introductory scenario.
All your enemies are "raider" nations. Well, the "raider" status enormously increases the endurance and the power of this nations, mainly if they are "officially" at war with you. The best way is to avoid wars, in the official way. Just try resisting their pressure, and cede the territories you are really not able to protect enough from their raids...
Remember this is just the introductory scenario.
When playing longer campaigns, you will see such nations aging and evolving little by little, so becoming less aggressive.