Hmm. i don't have a woman's anatomy, but i have a lot of experience as a bottom guy lol. i've been anally oriented since about 8 years old when neighbor friend showed me an enema nozzle and excitedly told me what it was for and how it worked. i had a crush on Him, so i immediately went home and found our enema kit, removed the nozzle and applied vaseline as He'd described and in it went. After that, pretty much anything i could find was at risk for insertion.
Ditto The NiceSadist, some training might be in order, i.e., start small and work your way up.
i suggest 'training' on your own to remove the fear factor, which can make you tighten up. By the time i took my first cock, i was quite ready (more than ready really, i wanted it badly). And i think that is key. Right now it sounds like your husband wants it badly and you are on the opposite end of that spectrum. Which means He may be a bit anxious to get there and you're a bit anxious that he not. It's a dance, not a race, and sometimes hormones can make a guy focused on the finish line.
While i'm gay, i grew up in a conservative religious culture that said i wasn't... so i did spend part of my life married to a woman. Though she never would have let me penetrate her analy with my penis, i did get her to a place of liking anal penetration with my finger, much to her chagrin i think lol. There was definitely a seductive process where i got her very wired by starting on her vagina and cltioris orally, when i got her to the place she was needing penetration, i inserted a well lubed finger vaginally while continuing to stimulate her clitorally with my lips and tongue, going for internal stimulation to layer it. As she would become more anxious for more aggressive penetration, i started to rub the rose bud of her anus with a well lubed finger, kind of using scissor finger (i.e., one finger stimulating vaginally and the other analy) till finally my finger slipped in analy with ease. i suspect if she had not had conditioned objections to anal, she would have been begging for it. As it stood, it was obvious she liked it by the explosive multiple orgasms she would have once i added that layer and slipped in during that process.
my point is, we all know sex is both physical and psycological and the body, emotions and mind need to be stimulated. It's hard for the body to open up if the mind and emotions are not on board.
On the physical side, i'd again suggest that individual, on your own training will go a long way in opening you up a bit (just like when you were a virgin vaginally and had to open). The anus and rectum are different and take a little more training, but they definitely can be trained. Not to be gross, but consider the size of what comes out, we know we can at least take that.
Good luck!!