How do I tell my client that I may not be a good fit for their needs anymore?

Tell her – honestly – that your current occupation is long term untenable because of your health and outlook, and reiterate to her that you don’t want to leave her in the lurch by quitting peremptorily (“or by dropping dead” you might add, if you have a dramatic flair), so give her a time by which you want to transition out of these projects. You may certainly suggest other avenues of collaboration for the future which suit her goals and your capabilities.

Be prepared to handle subtle and not-so-subtle inquiries into the nature of your unrevealed malady – and ready with how much information you want to divulge, if any.

@CWOTUS Yeah, that’s what I’m struggling with a little. Part of me wants to shed some of the stigma of ADD and otherwise being non-neuro-typical. The other part of me knows that ADD is misunderstood and I may be met with resistance “You don’t have ADD! You’re smart!” or “You’re just overtired/not trying hard enough/lazy/etc.”

The other issue is that I feel super awkward picking up the phone and telling her this. I know most people would not recommend email, but it would give me a chance to gather my thoughts and measure my words without potentially misspeaking. (Impulsiveness is another symptom of ADD and one I have to watch when speaking to people.)

Would I be an unforgivably unprofessional heel if I laid this on the table via email?

You’re not required to tell her anything about your health. Only that the job is affecting your health negatively (and vice versa, by way of acknowledging that you recognize that your recent performance has not met her standards or your own). Just leave it at that and try not to say another word.

Calling up and saying, “I find we are no longer a good fit given our different approaches to projects, and you would be much happier and more satisfied with someone else.”

I don’t think you need at all to go into details. I would recommend, however, that if you can give a referral to someone else, do so. And as you say good bye, outline the type of job you would do for her in the future.

How well do you know her? How much have you interacted? Would you feel more comfortable asking her to meet you for coffee to discuss a few things?

@Cupcake I’ve worked with her for about a year and we’ve interacted socially a few times. Unfortunately, she’s out of the state for the foreseeable future so I can’t meet her in person.

I sense she’s frustrated with me, (my symptoms have been worse lately and I’m seeking help) and I don’t want this to be an elephant in the room or make her do the dirty work of severing her relationship. I think that I could still serve her in valuable ways but maybe in a different capacity.

I agree with @CWOTUS .
If you can’t meet with her in person just now, that makes added difficulty. An email would be sudden and in your face. Maybe an email stating only that due to some health problems, you feel a need to discuss your continued work with her. At least then she knows you have something going on, and you know about it. That way, at least she doesn’t feel like you have lost interest in her as a client; if she is seeing a change in your performance now. That gives her a chance to ask, and to set aside time to focus her attention on the subject.

Do you know someone suitable that you can recommend? If I were in your position and as uncomfortable as you sound, I’d get in touch with a colleague, see if he or she might be interested, and then tell the client something like this:

“I can see that I’m no longer a good match for this project. The things I do best aren’t in line with what the project needs most at this point. I have a colleague whose special skills are better suited to this task. Would you like me to put you in touch with her and see if you’d like her to take it over? Perhaps I can continue to contribute in another capacity.”

I wouldn’t say a word about maladies, diagnosed or otherwise. Everyone has strengths and weaknesses; no need to label yourself.

If you do write a message, have at least two people check it for you before you hit send: one who is very sensitive to anything that might sound harsh (I call this a niceness check) and one who will fearlessly call you on anything that might be misinterpreted or misunderstood. Also let it cool for at least a day before you fire it off.

 
Just as an aside: Thirty years in the publications business made it very plain to me that writing and editing are two distinct sets of talents and skills and that very, very few people cross over successfully. To discover that you’re not an editor is by no means a reflection on your abilities as a writer.

@Jeruba

Exactly. I need an editor to keep ME on point.

One of my watchwords throughout my career was this: “Everybody needs an editor—including the editor.”

@Jeruba

I’ve drafted a short email. Would you mind if I sent it to you for review?

I’ll be glad to look at it. Can you copy and paste it into a PM?

Yep! Thanks.

You have a contract, I see from your details. What are the terms?

I would not talk about your ADD. I would just be honest about sensing her frustration and your interested in either working out a better arrangement for both of you, or if she prefers you are ok leaving the position. Say you really care about the work and the rapport when you work. Something to that affect. If you have a colleague you can recommend to her that might be better suited I think that’s really good, and your boss will likely still come back to you for some assignments when that person isn’t available.

Basically, honesty is the best policy, but you don’t have to discuss health issues.

She might already be looking to replace you, which would be raising her frustration level. Once people decide you’re not the right fit, they unconsciously make things worse sometimes.