I gave my cat's kittens away and now she won't stop crying. How do I help her?

About 4 months ago, I kidnapped a pregnant stray cat. She gave birth to 6 kittens, who will be 12 weeks old this Thursday.

Two weeks ago, I gave one kitten away, and she seemed fine. But yesterday, I rehomed 4 more, and now she's crying nonstop, searching for them. I feel horrible. Did I do something wrong? The kittens were still breastfeeding...

We decided to keep her and one kitten (the last one left), and he does comfort her sometimes, but I think he's getting overwhelmed by her sudden clinginess.

How long will she grieve/look for her babies? How can I ease her stress and pain? I feel like the worst person ever... I never had cats before so I don't know what I'm doing, did I really mess up?

Any advice is appreciated. I hate seeing her like this, she's my baby now after all..