Sorry to hear that your lady isn't agreeable to being licked anally. I love rimming nearly as much as I love being rimmed. It's an activity I wish more people participated in. It feels incredible and arouses me to no end...
Your girlfriend may be worried about the taste, smell or look of her bottom. It's a very common worry. Women are bombarded every day by ads promoting female hygiene, all telling us that it isn't normal to have a vagina that smells like a vagina. I suppose they want us to believe it's supposed to smell like roses, baby powder or a summer rain. There are specialized washes, wipes, douches, suppositories, scented pads and tampon. They feed on the insecurities of women and make them feel that any smell coming from their body is a bad smell that they should cover up. So with all of that to worry about, women can't help but feel especially worried that their bottom isn't a pleasant place to visit.
Making a woman feel comfortable with her tastes and smells will go a long way towards making her less self-conscious about her body and more likely to let you explore her nether regions. You might try giving her a warm bath and washing her or showering together so she feels clean and fresh. Massaging her with warm aromatic oils by candlelight will help to relax her and the scents will ease her mind about her own scent.
Telling her how sexy she is and how much her body turns you on will help her accept herself. If she points out flaws, turn her fears into something positive about what she feels negative about. I used to worry about a few scars I had after surgery, but while showering with a lover, he told me that the scars looked like butterflies. I hadn't looked at them like that before, but he's right. I saw myself as defective, he saw me as beautiful. It made a big difference in my life.
When you're making love to her, express your emotions and your pleasure, especially when you're going down on her. Let her know, by your moans how much you love her taste and everything about her. Encourage her to relax and enjoy the ride.
If she won't let you lick her, you may want to be a little assertive. Women sometimes pull away from their lovers when things feel to good and they feel like they'll lose control. Sometimes they want something to happen, but are afraid to admit it or afraid to let themselves enjoy it. They are tentative sometimes and move away, when they really want you to continue, but need your approval, acceptance or permission. Learn how to read your lover to know when no means no and when no means, I want to but I'm scared, shy or embarrassed.
Something I have used in the past with lovers reluctant to enjoy a good rimming might help for you. Buy a natural bristle paint brush, something small like you'd use for a painting, not a house. Buy some flavored lube, preferrably red in color. When you're in a playful mood and have time to take things slowly, dip the brush into the lube and very lightly paint areas of her body that you want to lick; her nipples, navel, labia, clitoris and perineum, if she's agreeable to it. Let her have the brush and treat you to the same fun. Try doing this several ocassions and dip the brush lower and lower each time until you get to her anus. Then lick her all over, enjoying the sweet, wet taste of her.