I really want to try anal but it just won't go in

Lately, my SO and I have been trying to try try Anal sex. I really really reallyy wanted it because the thought of it just turned me on sooo much. But it just doesn't go in. We use lube but it just doesn't go in. I think it might be that when the tip is almost in I freak out because I think that it'll hurt- at the same time it turns me on- but after a while of trying the moment just kinda dies. Any ideas as to why it won't go in?