Is this your first Terror Mission? Sounds like a Sectoid Medic or Leader is messing with you.
There are tactics to deal with these psi attacks that may help.
The enemy tends to attack your weaker (psi) soldiers, so attacks tend to fall on 2-4 main soldiers. This is a really good indication that they are Psi weak, and many use this as a marker to fire them after the mission to build a stronger force. I don't follow this method, but it is a way of approaching the game.
Anyhow, these targeted troops can be thought of as lightning rods drawing the enemy's fire. Once the attacks begin, try to disarm these troops if you have control of them, or give them non lethal weapons: stun rods, medikits, smoke grenades, etc. You can also try having your heavy weapons troops set their weapons on the ground at the end of their turns. Meridian did this to great effect in his SuperHuman Hawaii Challenge LP, and kept his troopers safe from enemy controlled rocket launches.
Combine this with aggressive search and destroy tactics, and you can start destroying the enemy's morale enough that they will panic, and very likely you will kill the Psi attacker and end the assaults.