My boyfriend wants me to have a with his ex. Should I?

I want to, but I am afraid to. I have not ever even touched another girl or even tried to be . I am also afraid that I will be overtaken with jealousy. I am very devoted to my boyfriend and we share a very strong and steady relationship, but I am afraid that if we try it, I will stop him and his ex- girlfriend in the middle of the act and say, I can't do it. We have already tried a with another guy, but it didn't work for us, so he wanted to try it with another girl- his ex. I don't know what to do! I thought that maybe if he doesn't kiss her, I won't get jealous. I want to make my boyfriend happy, but I don't know if I want to do this with his ex. The biggest part of my problem is jealousy. I mean, we had talked about it and I said that I would do a , but I did NOT know that he had his ex in mind. She is willing. My boyfriend had never touched her and I am not really sure if I want him touching her. I want to do this 3some, but maybe not with her. Help!!!!!!