My 3 months old puppy was bitten to death in front of my eyes.. So so sad...?

I was walking in the park with my little miko, which was a Maltese shih Tzu. Then a pitbull from another owner suddenly ran into my dog and bit it to until it died.. I tried so hard to get it let go but the stupid dog wouldn't until miko lost her breath.. Its her 3 months birthday today.. Ive been thinking of her special treat for today... But now I will never have the chance to give her all that.. I haven't gone back home since that happened.. I am so afraid and I don't know how to bear with the pain when she's not by the door wagging her little tail to me when I open the door.. I was so happy the day before when the vet said she was very healthy.. I can't help but blame myself for taking her to the park at that time..!! I'm an idiot..