It might (might!) help to remember that it IS a deliberate kick in the guts. But she's just angry and trying to hurt you - she probably doesn't mean it. Teenagers are very self- obsessed focussed, and angry teens have a stunning ability to overlook how their actions and words affect others. Also on the bright side, she is probably hitting out at you because you are a 'safe' person to be angry with: she knows you will still love her, even if she's utterly foul. My son actually told me this (in calmer moments 'after a storm' he's actually quite self-aware).
My best friend had a similar situation with her eldest (almost ten years ago now). When he was about 13, and had just grown bigger than his mum, he hit her in an argument, and then 'phoned his dad to collect him. My friend's ex was almost gleeful, and instead of behaving like a decent grown-up and saying "no, sort things out with your mum", he was round in about 20 mins. The boy moved out for a while, and my friend was heart-broken. In the end, he grew up enough to realise he had been at fault, and to recognise his dad had exploited the situation, and he moved back. These days, he lives at the other end of the country, but when he comes home to visit, he stays with his mum, not his dad.
It is really hard when you have an ex who behaves badly like this. But the good news is that if he can do this, he isn't mature, and the chances are he will fail to deal with the first conflict he has with your daughter. She might be home sooner than you think. Fingers crossed. Even if she isn't, she'll almost certainly come back to you in the end.