One of the issues often faced with telling someone about a psychological condition or disorder is that many people have at best a confusing idea of what the name (the label) of the disorder translates into actually meaning. And generally different individuals experience different degrees and expressions of related symptoms. Furthermore, a label is a generic term that's simply not personally relatable in and of itself, unless that person is already familiar with someone else who it applies to.
Disorders with names that in the vernacular mean something different than their clinical application are prone to this, as are disorders that are often the focus of lampooning or other similar treatment, which only magnify the misunderstanding that is their basis.
Personally I've always found the best approach is to start with what's going to be relevant to your interactions with the person whom you're talking to, and allow that to provide openings for further discussion on the more general topic/diagnosis.
- Places the focus and context on you.
- Makes the topic immediately relatable, because you are the actual focus of the topic, rather than a generalized concept.
- Helps avoid confusion related to prior or different understandings that aren't applicable to your situation: people generally cling to what they already know (or think they know), and the easiest way to change knowledge is through providing experience rather than merely telling someone that something is different than their current beliefs.
- Is a sincere show of openness and trust.
- Invites further conversation related to you and your experiences, clearly signaling that you're open to talking about it in that way.
I still have sensory issues that can be problematic. While I don't tell everyone about my condition, I could never seriously date someone that didn't know. It is almost a guarantee that at some point while with her I will experience difficulties that I can't control because I'm autistic, and if she is aware then she'll be better able to understand and deal with the situation. To be clear, I'm fully self sufficient (job, apartment, etc...) and not looking for someone to take care of me. I just don't want to scare her the first time she sees me experience sensory overload.
Why not start here? You have a medical condition that's generally under control but can cause certain things to happen when you experience certain environments. You can offer details on that. Be open about how it makes you feel when this happens, and it's ok to be open about how you generally feel about discussing it.
Part of a good relationship is establishing trust and finding just how far those levels of trust go when you make yourself vulnerable in some ways to that person, and while telling someone you have a clinical disorder and terming it as such is certainly one way of doing so, I've always found that leading with personal experiences and then following up with the explanation of something being classified as falling under a certain clinical label leads to both less confusion while also clearly signifying that you do want to and are open to talking about this in a personal context rather than just as a label.
Labels exist to help facilitate a quick (imparted with just that label), common, presumably shared general understanding in a way that is by necessity both imprecise and impersonal. They're at their most useful when the point is, ideally, to avoid having to give an entire definition of something when just the label itself will suffice. But the point here should be about you and your potential partner and your lives and how this signifies in them, and your ability to communicate between the two of you on personal matters like this… not just a generic term.
Starting with the personal aspects of living with your condition help place the resulting conversation in the context of it being a personal discussion of you and your life and not just a general clinical label that applies to you in some way.
I've never found a specific wording or approach that necessarily makes this easier for me, but I have found that generally approaching life details like this can be helpful in turning the discussion into an honest conversation that focuses it more personally rather than trying to start off awkwardly as to whether the other person feels you are opening things to deeper discussion or not, or even defensively against either stereotypes or other misunderstandings (or worse, finding out later that the other person claimed to understand but held such misunderstandings but didn't even voice them, and they colored subsequent interaction).
This is the type of discussion that I would personally bring up sooner rather than later, if I felt like I was to a point where I could tell that it was someone who would make a good friend who I'd want to spend time with regardless of whether or not the relationship developed further romantically, once at least some more general personal topics had already been broached.
I think at the point in time where you judge from how things have been going that you'll be seeing the other person enough more for it to be good for them to be aware of your condition simply due to the outcome of symptoms of it is probably the right time.
Speaking perhaps more personally, I don't think you're hiding anything if that time does come later rather than sooner, and I also personally would feel that anyone who judgmentally considered you to have been "hiding" something simply because you have developed strong coping skills and it's not superficially apparent that you are afflicted with anything… might be someone you're better off not being in a relationship with. With that said, it's usually best to get certain details out in the open early: if you're afraid that it will affect a relationship, it's better to know sooner rather than spend too much time with someone only to find out they're more concerned with a label that's merely one of many that can be used to describe you than they are with the actual person they've been spending time with.