How to get used to anal

My bf and I have tried anal a few times before. One time I handled it pretty well and it felt good, but every time since then the sensation is absolutely unbearable for me, even with lube, trying to breathe and relax my muscles, going slow, etc. He really wants to try more but I get scared!!! I may be psyching myself out because we did it once before and I enjoyed it. How can I get used to the sensation and get comfortable with it? Thanks ladies!!!