I physically can’t do anal????

Like I literally can’t. My Boyfriend and I have tried soooo many damn times but my ass just won’t open up?? We have tried for almost an hour just to widen my hole but it’s always tense and I never know how to relax it ;( even when it feels relaxed, nothing really fits. I’ve only been able to for my finger in but only half way. It sucks so much because I really want to try it and so does he!! Any advice ladies?? Is it actually impossible for some to do it?