Jer, The card is a reminder of what she once had w/you. It makes her feel guilty right now and she doesn't want to feel that way, especially if you wrote some nice things in the card. I would venture to say that the card is somewhere safe at the moment. Some day, she will look at it, especially when she's alone in the house and has a lot of time on her hands.
As for her being happy, she's miserable in many ways. She wants you to move out and yet in the back of her mind, she doesn't. To be perfectly honest, a mad hornet is better to deal w/than a MLCer because you know exactly what you've done to irritate them.
The best thing you can do is continue as you have been and find joy in your children each every day. Right now, she can't stand the fact that you aren't crying, begging and pleading w/her, but most importantly trying to have discussions about the relationship. You aren't giving her any justification for what she's done or continues to do. The best revenge is to live well and be happy.
One day, when your children are older, you, and hopefully your w, will sit down and tell them what happened...but for now, they are so young and truly don't understand what is going in w/her.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.