Strange that to you lost the eastern territories. Militia hoplites can actually stand against early Parthian/Armenian armies if they have numerical superiority. Keep them on loose formation and have a general around to pancake the cataphracts when they charge. Be wary of the yellow doom moving upon you from the south now. I'd suggesting holding on for dear life to Antioch and Damascus, otherwise you will be unable to train elephants which are a great advantage to your armies.