Trying anal for the first time

So last night, me and my significant other tried anal for the first time. I want to emphasize "tried" lol. He was able to get a lubed up bullet vibrator in and fingers but once it came to him trying to put his penis in, it wasn't happening. I was trying so hard to relax but my booty was NOT letting him in. We had a shit ton of lube and he was going easy. Idk why but my butt was just like "nope!". Do you all have any tips on how to relax the fuck out?? My mind and body were relaxed but my butt just did not want anything going in I guess.