First, it's not your fault when your cat left. I have suffered it 5 times and now, after being told many stories and seeing them leaving and coming back to my friend's home, I realise that they just serve their instinctive desire. But if something bad happened, it's not your fault too. You can't keep your eyes on him/her 24/24. Remember it. Hang on, this feeling will be over soon as it's been all the time. Remind yourself of happy moments you had, my dear.