When they're young, they feel every bump along the road so much more intensely than they will when they're older (and wiser). As their mothers, there's very little we can do for heartbreak other than be there if/when they need to talk (which they do, even if it is days later once they've processed), allow them access to their supportive friends (huge help), and carry on as normal (because this is a natural part of their growing up).
My 15 year old son had his heart broken for the first time when the dog he'd grown up with had to be euthanised last October, on Halloween. He thought he'd experienced heartbreak before, when his first ever girlfriend ditched him at 14 because he had a facial tumour (she's very shallow and led by appearances rather than substance) - but nope. He said that was nothing in comparison to the death of his dog. He cried, I held him (ignoring my own grief), we talked, he went out with his mates and they reminded him that he needs to remember the good times, and got him to talk about all those memories he has of his first ever "best friend". He's still grieving now, but his friends help. He's also got another girlfriend, too, so I expect there'll be another bout of grief when that ends, but he knows (sadly) that fleeting relationships coming to a natural end, hurts less than death.
Your 13 year old will get over this. Yes, it's true, that we never forget our first loves - and doubly true that we experience maternal rage when someone hurts our babies... so your anger is natural. But you can't do anything other than remind your 13 year old that this? Is but the first step on a long road. Let them be bolstered by their friends, who will "get" the grief, because they're the same age. Cook their favourite supper, do something fun with them, listen to them, hold them, let them feel how/what they want to. But remind them, too, that someone else will catch their eye, and they'll be older and wiser then, because of this break up.
And don't ever take your natural anger out on the one who rejected your baby. 
OP, because this is a very long road to be walking.