To validate is to do something like this...imagine a conversation between you and your W. This is how it would normally go WITHOUT validating.
W: I really think our marriage is over. We have not been on the same page for a long time and I wonder if we will ever get there again. I don't really even feel like trying anymore.
You: What do you mean! We are always on the same page. Our marriage is NOT over. I KNOW we will get there again becasue I believe in us and I love you. I think if you give us time, we will be ok.
Now, on the surface, that seems ok. You didn't get mad really, you simply stated how you feel but in the course of that, you invalidated how SHE feels and in turn, made her feel "wrong" for feeling that way.
Here's how the same thing would go if you were to validate her:
W: I really think our marriage is over. We have not been on the same page for a long time and I wonder if we will ever get there again. I don't really even feel like trying anymore.
You: I understand how you feel. I know things have been less than good lately for you. It makes sense, given all that we've been trough latley for you to feel like there is no hope. I'm sorry you feel that way.
The key to validation is not necessarily to agree with what she says because you CAN validate AND disagree. The key is to let her know that you heard what she said, and accept her right to feel that way. If you want to then disagree, fine, but in your case, probably better to just validate and then move on.
Again, you just don't want to make her feel that YOU don't think she has the right to feel a certain way. By validating her feelings you make her feel safe to express herself to you.
Remember, you can validate without agreeing.
Once more...
W: I really think you're an a$$hole for how you've treated me this past year.
You: I understand how you would feel that way.
You could then go on to appologize if you agreed with her, or you could simply say "but I don't agree with you." Don't defend yourself, don't get into long explinations. Just simply validate and move on.