Persistent dreams about trying to have anal sex but it "never fits in" (NSFW)

Okay, this one is a bit weird, so I made an alt. I'm a 22 female and have a fair amount of sex dreams (about a couple a week, sometimes more). Usually these dreams will involve vaginal/oral sex and will be enjoyable, but semi-reguarly (maybe every couple weeks) dream me will decide to try anal, however no matter how hard I try I just can't manage to fit the penis/object into my ass. It's kind of frustrating, a bit like those dreams when you're trying to run away but you're legs aren't moving. Often the penis will just go floppy and i can't get it in. What on earth could this mean, and why do I dream of this so often? In real life I don't do anal as I've tried it a couple times and didn't enjoy it, might be relevant. Me and my boyfriend have a satisfying sex life so I don't think its some kind of sexual frustration thing, and my vaginal sex dreams are normal.