Panic Attack

Today I woke up with a panic attack, I don't quite know why but i had a bad one, i could barely breathe.
I knew my ex and his G/f was leaving today and i guess I was a little sad to see them go, we had so much fun while they were down and the kids really enjoyed it.
I made my Ex Tell our oldest, Dakota that he was leaving and that he would be back for his birthday in August. Dakota shook his head in agreement but i don't think he understood exactly what he ment at that time.
The whole way to work i was very sad, sad to see them go, it felt like a part of me was gone.
After I came home with the kids, Dakota saw his dad wasn't here, he said Where is my daddy at? I told him going back to Virginia where he lives and works. Dakota got very sad. I talked to him and told him that His Daddy loves him and when daddy can he will be back, but he has to go back cause thats just where he lives and works and this is where we live and you go to school at.
Then after dinner, Dakota said I don't like daddy, I love mommy. That made me very sad again. I don't know what to do to make him understand, I mean how do you make a 4 year old understand something like this?