Welcome to the site.
I am sorry to know that you are having a tough time dealing with therapy sessions. You see the reason behind therapist asking you two to say something good about each other at the end of session is that he wants you to reach a stage of harmony with each other or at least a state of being cordial with each other which is important for rearing a child together as two individuals rather than as a couple. You see eventually this sort of exercise would help you two to respect each other's opinion and boundaries when it comes to co-parenting a child and also it would help the two of you to refrain from trying to cut each other down to pieces when you are not in full agreement with one another and maintain a dignified stance in such cases.
So. while you do not have to say something good about her by just making it up instead you should invest some real time in remembering her qualities or good points and bring them to the table. This will inspire your ex too and the cordial relationship will flourish which will be for the betterment of your child as a child would get hurt or depressed seeing his/her parents fighting with each other.
If you think that you are unable to come up with good points about your ex during sessions, then make sure that you mention the same to your therapist, this is because he needs to know about this and he shall tell you what you can do to overcome this situation.
I hope this makes sense and gives you some perspective.
Wish you all the best.
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