It will almost certainly feel really horrid at times, BUT if you don't go, and put on a happy-ish face, it could give the impression that you can't cope with him moving on, and the corollary to that is that people are either going to pity you and presume that you secretly want him back and are broken-hearted, or think you are avoiding the dinner because you are so mean-spirited that you don't want to see him happy. I wouldn't want to go either, but I would go, and be friendly and gracious, take the measure of the new partner, and if I felt that they are rubbing my nose in it a bit, then I would mention that I had joined an internet dating site (the norm, rather than a shameful thing now), but that while I was going on the odd really enjoyable dates, it was still early days for me, and I was enjoying my own company too much to commit, thus implying that they were rushing it! I would suggest though that you really ask yourself why you feel it would break you to see them together - if you are genuinely OK with the split, and still care for him, then you want him to be happy, don't you, even if it means being with someone else?