The way to avoid this is fighting different factions, in my Sauromatae campaign I reached 201 and now I am close to raiding all remaining large and huge cities of the Seleucids and cripple their military production.
I also finished off the Saka and Baktria is entrenched in 2 full-stacked cities.
And now my invasion into Thrace and the Getai lands(now owned by Macedon who pushed the Getai north and west into Sweboz territories.
The Aedui almost finished off the Sweboz but as soon as I allied with them the Lusotana and Romani started a full out attack on them (seems that if you ally with someone they're marked for death by the other AI factions).
This turned into an AAR...
Anyway to avoid burnout try fighting varied factions or play 2-3 campaigns in parallel with different factions in different places of the world.