NJAPF if you are expecting empathy and support there are plenty on Gransnet who would rather judge and reprimand, so be prepared. As for Mumsnet, well...
I do have a lot of sympathy for you and do not agree with those who seem to think that losing your temper is not 'adult'. It does not do children any harm to see anger, but they also need to see it is contained, that the adults can handle it and not get out of control. And of course it's good for children to hear an apology, where one is due. I would say one is due here, NJAPF, because the thing to do is of course to call the behaviour horrible, not the child. To be called 'horrible' leaves no hope, to be told one's behaviour is 'horrible' means one can improve and learn. It sounds as if you acted under great stress and were very tired, so it's understandable, but in my view it was not advisable and needs an apology for the sake of your relationship with your grandchild, and the child's welfare. That memory could have a significant effect, but you can put good memories to balance it out. I am not a great fan of the ultra-gentle parenting that's promoted these days. I'm not sure what sort of adults will be raised - sometimes it works, no doubt with certain subtleties and circumstances, but the danger is it can create an entitled, selfish human who is simply not equipped for the demands of life. I also think that it is no wonder so many people are suffering from stress and exhaustion, when both parents are working full time and then have to be all sweetness and light for badly-behaved kids - or suffer guilt and criticism. By the way, I'm a counsellor with 37 years of experience, mother of four and grandmother to four, so I say none of this lightly.