She'll get married soon and I got invited.

My ex who is also my best friend is getting married two weeks from now and I got an invitation from her. I’m having second thoughts on going since our break up did not turn out well. We broke up five months ago but I’m still stuck in the rock bottom of pain. She is getting married with the man who works at a foreign restaurant in our city and whom she cheated me on with. I know I should be happy for them and I’m trying to be. Should I come? If I should, what would I say? I don’t think saying “Congratulations, I’m happy for the both of you” will do good for me. I’m just lying to myself if I come and say that.