I want to but I'm scared!

I have only been with one person for 11 years and we have had sex 100s 1000s of times doing all sorts of things, but I get this feeling I want to sex someone else. This dude I've known for years he wants to and I want to but I'm scared, we get so close and I feel like it's my first time again but it's not and I back out. He says it's ok he understand but lately I can tell he's getting a little frustrated. I really do want to but I don't know how to go about it... I'm not the shy type with my bf but I am with the other person. Please don't judge me because I want to cheat, I just need to be with someone different for some reason... I want to. I don't want to get married and only have ever had sex with the one guy my whole life...