Unfortunately, all that information goes into a databank called the Medical Information Beureau (MIB). Which means, even if you don't say you have ADHD on the next application, the insurance company is going to look you up and see you were previously denied and reject your application based on the MIB report.
Also, if you intentionally leave off information, even if they don't catch it, that's called misrepresentation, maybe even fraud. Very bad. Very very bad. Don't do it.
How long ago did you have insurance coverage before? Were you able to get COBRA after that or has that period passed? If you are able to get COBRA still, I would jump on that right now if I were you, regardless of the cost, then worry about converting that into an individual health plan tomorrow or whatever.
If that's not an option, I found a website with some great information about this subject that might help you, but I'll have to send it to you in a Private Message because otherwise the moderators will take it down.
I hope this helps. Best of luck to you.