Some of this is similar to avazula's answer (which is great!), but I wanted to share the approach that I've used too. The main differences I think is that my mother is in the second category (tends to worry, rather than dismiss) and that I did name it and make this a serious conversation. In my case, since my mother had follow-up questions, and because she was seriously worried, I think it was good to make this a serious conversation and give her that info, instead of trying to be nonchalant and less specific.
- Explain that you agree, what she's noticed is an issue, only that issue is anorexia (and not just being picky/unhealthy/etc.)
- Explain what your plan is to manage this issue
- Be prepared for follow-up questions, and decide how you want to talk about this in the future
My experience is with telling my mother that I have dermotillomania. The term isn't very well known in my experience (I first learned about it by chance from a comic I was reading), and the actual details of behavior, severity, and causes can vary a lot by person -- kind of like how you had to clarify here "it's not that type of anorexia, it's this other type".
In college it got pretty bad due to stress, to the point where she kept mentioning "that rash you've had for months".. but I was afraid of scaring her by explaining what it really was, so I just tried to deflect or shrug off comments. Eventually on my own I decided I needed to talk to a professional about it, and made an appointment with the counseling office at my college.
The next time I called home, I told her about making an appointment. I called because I didn't have plans to visit soon; otherwise, I probably would have done it in person, when just the two of us had a quiet time to talk.
The conversation went basically like:
Me: [finished talking about the usual random news]. Also... I wanted to let you know, I made an appointment with the counseling office.
Her: Oh.. what for?
Me: Well, you know that "rash" you've been mentioning? I, uh, actually it's this thing called dermatillomania, which is [definition, brief explanation of how it manifests in me specifically and how I realized it was an issue, etc.]
(but with some crying from my end because it was tough to say everything out loud :P)
Since you already are seeing a professional, you could instead mention your plans to talk to them about it. My mother asked a question or two, and mostly she seemed relieved to understand what was going on and know that I was seeking help for it.
As it turned out she was about to send me to a dermatologist... so I guess it was good timing. But my point is, your mother is probably already worried, since she's noticed your symptoms - so you can view telling her as a way of easing those worries, since now she has the truth, instead of leaving her in the dark to guess and come up with possibly even scarier ideas. This is also why being specific helped -- when she kept mentioning a "rash", it was because she thought I didn't notice or know what the problem was, and wanted to help. Since I told her, it reassured her that I knew what was wrong, so no need for her to keep trying to diagnose the problem.
It also helped that I didn't only say the name of my issue, but talked about how it explained my symptoms. That way, even if she looked up more info about it later, she would know I wasn't at the worst-case scenario (which can be very bad for these types of issues), it was "just" some less concerning stuff that had fairly established ways of being treated.
Hopefully, knowing this will reduce the comments she makes about your food choices. But you can still ask that directly! During this conversation I told my mother that her comments about my skin were really uncomfortable for me, and she stopped.
Also, be prepared to talk about it more than once, since after she has time to think about it she may have more questions. But if there are certain things you definitely don't want to talk about, I would mention that up front so it's clear. I didn't do this, and then she asked about my counseling sessions, which I didn't really want to discuss with her, so that was kind of awkward. This was the first time I'd really talked about serious issues with her though, so if you've already established some healthy boundaries around this sort of thing it might be less of a concern for you.