Dude. “convince” her to “fuck her in the ass”. Really? It sounds like you pulling one over on her. I am guessing part of the problem she doesn’t want to give you her ass is that maybe she senses that your motive and tact kinda suck.
Sometimes this shit is just too easy to stand out.
If you are taking a girl’s anal virginity, you might want to take a more caring approach. Since you are adults and she is your girlfriend, I will assume you are a good guy who maybe misspoke a bit, and I’ll just give advice on how to not make it the worst experience of her life when you do get there.
First, I’d first learn how to make it the least painful and a more pleasurable experience for her. Talk about it with her.
When women do a water enema first, two things happen: one, the mental “I wonder if shit is gonna come out” factor is not a concern and two, her butt has already been through some action so it’s less painful. The enema is key.
Next, lube. Lube her ass. Lube your dick. Lots of lube.
Next, finger her ass. For lack of a better term, stretch it out. Play with it. Go one finger while you eat her pussy, then two, then the peace sign. Move around a bit.
Last, just smash it in there. LOL. Just kidding. Lube up again and when you are about to actually insert a dick inside a woman’s ass, you gotta be kind. Push the head in, wait for the pop once it’s in, and hold it there, and wait until she says it’s OK. Then push in but wait again until she has adjusted. Once she is OK, in a couple minutes of slow fucking you can fuck to your heart’s desire, but you have to be gentle with her at first. The ass expands so allow it to.
Also, buy her pretty little prince plug, ask her to send upskirt pics, or call her in the middle of the day when you know she is at work and ask her how it feels. Is her pussy wet? How does it make her feel? Tell her how fucking sexy she is and how you love that she is filthy for you and only you.
No matter what – make it about her. It’s not about you being able to “fuck her in the ass”. Just my opinion though.