Adult students aren't as prevalent in the USA as they are in other places I'm used to studying. The first university I went to, in my home town, was comprised of 1/3 of the student population being `mature' (over 25). It wasn't at all uncommon there for people to take some time out after school to travel or work for a while - to gain some life experience basically - before figuring out what it is that they wanted to do. Sounds like your interest in psychology is more recent (that wasn't the degree that you started?) so you can certainly make a case for your having some time off to figure out what it was that you wanted to do before making the decision to go back.

I wouldn't personally mention mental health issues. My feeling on that is basically that it really isn't anybody else's business. So it isn't that I'm ashamed or embarrassed or whatever, it is just that it really isn't their business. If you have mental health problems while you are studying then (hopefully) there are good counseling services etc there that you can access. There are many people who are undiagnosed who struggle and I so wouldn't worry particularly about whether you will find yourself struggling more or less than them.

Applying for a clinical psychology or doctor of psychology program will be a while off yet. Your history with depression MAY be relevant (you may choose for it to be relevant) when you apply for that... Or it might not be. It is some time off, yet, so I wouldn't worry about that, particularly.

That being said some people do prefer to disclose mental health issues. I don't - but I do appreciate that that is a personal choice.