I've been running a campaign for over 2 years, and the party consists of five level 17 characters. The problem is that they don't take any issues seriously. They skate through any challenges I throw at them, and they are powerful enough that there are few monsters that would actually be a challenge for them. They also don't take any of the villains seriously. They take things too casually, as though they are invincible. I understand that my goal is not to defeat the players, or view them as enemies as the DM, but they have had things way too easy in this campaign. I'm looking for ways to psychologically torment my players, but I have to be careful how I do it. Trying to villainize the party doesn't really work, as they claim they haven't done anything wrong (They have committed plenty of crimes, although nothing too terrible). Does anyone have any ideas on how to add a new element or layer to this campaign that will force the characters to make tough decisions or to have to rethink how they view the world and how they view themselves?

That's inevitable for high level characters. They have so many abilities and powers that nothing is a serious challenge anymore, so there is no need to take anything seriously. Even if they die they can be easily resurrected again. Tier 1-2 is where you can have serious risky grim-dark themes. Not in tier 3-4. 

I understand that it is very difficult to torment or traumatize them in encounters or in grisly-dark themes. But is there something I can do to torment them psychologically, like with how they interact with the NPCs around them, or being forced to make very difficult moral decisions, or being forced to pay for their past mistakes.

You don't torment the players by attacking their characters.  You torment the players by attacking the people their characters care about. 

Now it may be a bit late for this, since they're already level 17, but here's what you should have been doing all along:

1. Introduce an NPC that your players fall in love with.

2. Kill that NPC. 

3. ????

4. Profit!

- introduce a mysterious npc that keeps showing up wherever they are. never interacts with them, just someone with the same, memorable description. if they notice and kill the npc, it keeps showing up. if they finally actually talk to the npc, have it explain some connection to the next threat/bbeg, and then never show up again, unless they try to find it in some specific way/with a specific spell.

- false hydra. (unless they're metagamers, in which case this will be harder to make work).

- magical phenomenon that switches all of their classes but keeps their ability scores as a small-range environmental effect around a big bad. the wizard has all of the abilities of a level 17 fighter, but a 9 strength score and no real weapons. good luck!

- the classic: give everyone in the party a note that says "you are not the doppelgänger, continue playing your character as normal". they'll all start suspecting each other. if you want to make things really fun, actually do include a doppelgänger, tell them that their goal is to get the party to follow them to the doppelgangers' lair and kill them all, in reality the ganger is leading them into a trap. if they find out who the ganger is and kill them, oh well. they now have to go on a quest to save their kidnapped party member. (again, harder if they're metagamers, bu could be really fun).

Ooh, I really like those. Thanks!

Where is their home base? A city, town, specific neighborhood, or even specific tavern? Hobgoblin army invasion! Put them in a position where they need to choose between what to save and what to leave, they can't be everywhere at once! 

Um, if you want psychological torment, I have an idea. I've wanted to do this with my players, but have refrained from it for now. The plan I had was to put each player one on one against an illusion that reflected their characters (or maybe the players, I was kinda crabby when I came up with this) fears. It is supposed to startle at the least and terrify at the most. And you can scale it to what you need it to be. It's simple, but might be effective

I don't mean this in the snarky way it might come across, but you might also consider asking them? That is, you don't have to use what they say if anything, but if the problem is that they don't seem invested, maybe they've noticed it too and could give you a starting point.

Part of it is that a Level 17 party kinda should feel at least pretty tough if not actually invincible; they've worked for it.

But some other options:

• A similarly powerful party of rival adventurers appears seemingly out of nowhere, beating them to the punch on several quests. While incredibly obnoxious, the rival party is not openly hostile or dangerous, but there's something suspicious about them...
• They have a prophetic vision of themselves in a fight lopsided enough to be unwinnable even for them (probably be upfront that it's a dream sequence, but play it out so they can see that they'd be in trouble if it were real), and they have limited time to prepare for the real battle, either by increasing their own power or weakening the opponent.
• A seemingly ordinary adventure is a trap that somehow poisons or curses them. They probably have spells for that, but "Special curses need special cures" is usually an uncontroversial bit of DM fiat. Again, ticking clock to save themselves.
• If they're not receptive to being criticized for things they actually did, trick or frame them so they're at least motivated to prove their innocence.