Planetes is my favorite manga, and deals with a lot of psychological issues in its characters, but it's more about the characters and their interactions, I suppose. (If you end up looking for a good action series, Vinland Saga rocks hard. Makoto Yukimura is awesome.) Nijihara Holograph and similar works by the same artist/author are a collection of short stories that deal with the psychological problems some of the characters have, and I found it to be a pretty good read, so you might want to check it out. All the manga I've mentioned is under 5ish volumes, though (Planetes is split in the U.S. release, which I own, but kept as four volumes in the Japanese one, though it's still a pretty lengthy read), so if you're looking for a longer series like Monster or MPD Psycho, you might want to look elsewhere, I guess.

MPD Psycho is a pretty good choice (if you're not easily grossed out), and Monster, too. I haven't gotten past a certain volume on either of those, though, because I was a bit too busy to read when they were being scanslated. But that's some good, quality manga nonetheless.

I'm sort of drawing a blank, but I guess this might help you, although some of the manga that appears on that list might not even be worth mentioning.

EDIT: Forgot Akira, which is like the pinnacle of psychological manga (if you haven't read it already). It's only six volumes (in its U.S. release, which was awesome, by the way), but each volume is preeettttty big so it'll keep you satisfied for a while. When I get enough money, I think I'll get the real thing. Although Dark Horse reversed the comics in print, I still think it was a great quality publication.

Try reading Bokurano. It's somewhat of a mecha series but not exactly like Gundam or other series. You can also try watching the anime which is pretty great, but makes some changes to the story which makes the manga much better.

EDIT: Ugh, I'm too use to reading Right to Left, are all Comics/Manga changed to Left to Right when they're translated professionally?

Very few books, as Christophomicus said, are translated left-to-right now. But back in the day, when manga was still a bit new to the states, most manga was published left-to-right because they supposed a wider audience would read it if it was printed in a way that was more accessible to readers. I've got a realllly old copy of Card Captor Sakura that is printed backward, and a bunch of the Inuyasha books at my school's library are left-to-right as well. Nowadays, people have adapted to having "pure" manga, so you'll be hard pressed to find some reversed manga. Anything that is is probably Korean or something. I think they read left-to-right (?).

Uh, anywho, Akira was published by Dark Horse back in that time period, which is why it's backwards. I think.

I rather liked Dead Men Wonderland. It's pretty phsychotic.

What brought about you reading comic strips backwards?

Wunderbread, Planetes is awesome so far and it does seem even more like something I was looking for. Thanks.

Hey, cool. Glad you like it.

I rather liked Dead Men Wonderland. It's pretty phsychotic.

It's taking awhile for it to be translated, though. And it feels a bit more action-y to me. Still psychotic, though, but not in a "mental-intrigue" sense. More like a "WTF" sense.

Only manga? I know some psychological anime, such as Paranoia Agent and Paprika. But if you want only manga, you can try...uh...Psychometric Eiji. I found the art gory and disgusting so I didn't like it very much, but you might.