OK so first understand that validation is a learned skill and is very difficult to master, so don't beat yourself up too much but that wasn't validation. Now I'm not saying your response was wrong or harmful by any means, but I do want to help you to understand the difference. Validation is first seeking out her FEELINGS and then ACKNOWLEDGING them. You are not agreeing/ disagreeing/ arguing/ negotiating/ explaining. You may not agree at all with what she is saying, you are simply acknowledging that they are her feelings and they are valid. Example:
W- I know how hard it is to juggle the kid's schedule. It's been my life. You- It sounds like that was frustrating for you, is that how you felt? (seeking out her FEELINGS- mad/ angry/ disappointed/ frustrated) W- I enjoy doing things for the kids but it made me angry that you were never there, that I had to do it on my own. (note her feelings were actually "angry" and not "frustrated" like you suspected, so you adjust your response, see down below)
Now note what 99% of guys will do, right here, go into self-defense mode:
"But I just took them to the movie last week, and I took them to school 2 weeks ago and picked them up the day after that, did you forget all that????" This is a terrible response because what this says is "I don't care about your feelings, they are wrong and I am going to tell you why."
Instead a validating response would be:
"I hear you saying it made you angry that I didn't help more, I can understand why you would feel that way." Again you are not AGREEING with what she is saying, merely supporting her feelings.
When you learn to properly validate guess what happens- 1) you immediately put her at ease, it reduces conflict. 2) she feels like you are actually listening and seeking to understand her. 3) she'll reevaluate what she said. My girlfriend will talk in absolutes- "you NEVER blah blah blah" and when I listen and validate she will back down and say "well that's not true, of course you do that, I am sorry I'm just having a bad day." It is a more useful tool in a happy relationship because your SO wants to find balance instead of fighting. It can be frustrating when dealing with a WAS though because sometimes they see no good in you because they're filtering it out to justify what they are doing. So you have to keep working and working at it.
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712