If you want to talk to your mother about the issues you're facing right now, I would not advise you to use the word anorexia.
Please be assured: I am not questioning your diagnosis in any way. I recommend not to use the word "anorexia" because it comes with a lot of stigma and misconceptions and she might picture your situation in a biased way.
A few years ago I suffered from depression. I had disclosed my issues to my mom and she tried her best to make me feel better (she thought I was "simply" going through a rough time). But as soon as I was officially diagnosed with depression and told her about it, she started to hold on to fantasies she made up with her own prejudices about the disease. I felt dismissed, unsupported and alone in a time where I really needed to be believed and loved by my relatives. Parents usually have a really hard time bearing their children's pain, and they may either dismiss your condition (because they can't deal with the idea you're hurt so they reassure themselves by telling themselves that you're okay) or panic about it (because they don't want you to suffer and they want to do everything they can to prevent you from being more hurt). My mother is from the first category, and even if I don't know if your mother falls into one of these two, I think it may be helpful to announce your issues by mentioning facts only and not using words that carry a lot of stigma.
You say she's already worried by your condition. I think a way to reduce the shock of your announcement is to mention it in a neutral way, and to possibly tell her how you've planned to tackle the issue as well. That way, she'll know you acknowledged something she already noticed (you say she told you she was worried by the fact you eat the same food every day): acknowledging is the first step towards healing, and she'll be reassured by that. You may want to talk about it after some banter or casual conversation, so that it shows it's not that worrying (some people may think it's insensitive of you to announce something bad in a light way though. I advise you to adapt the context depending on you and your mother's personalities).
Something along the lines of
[...] by the way, I wanted to tell you. You've told me before you find it weird that I always eat the same thing. WellI've been thinking about it, I recently realized I may have an eating disorder and that I should act onthink it may be a problem that I'm not interested in eating anything else. I already planned an appointment with [professional health practician name]. (Thanks for pointing it out to me!)
could do.
A few things to consider:
I'm not suggesting you lie about an appointment. I think seeking professional healthcare is very important in your situation (please do so!), both because mental health issues are serious and need support to be dealt with, and because your mother will be reassured if she sees that you're trying to address these issues.
Thanks for pointing it out to me! -> This is optional and depends on your and your mother's personality. If you're both comfortable with such expressions, it may show love and gratefulness to your mother and make her realize you listen to her and don't want her to worry about you.
eating disorder already is a word that comes with its share of prejudices, but it's easier for people to picture what could be an eating disorder whereas most people think anorexia only is about eating lettuce once a day and forcing yourself to throw up. It's an understatement that reassure people but still conveys the issue.