Looking deep into the spell

The spell states that it is an illusion (figment) [fear, shadow] spell, meaning that it has two parts the [fear] and the [shadow]. The spell goes on to describe that the shadow part has a quality that obscures vision and provides concealment, that much is understood. The fear portion of the spell is the "Illusion of misty vapor inhabited by shadowy shapes that arises around you". The (figment) tells us this "A figment spell creates a false sensation. Those who perceive the figment perceive the same thing, not their own slightly different versions of the figment. It is not a personalized mental impression." This tells us that the spell does not change, and that the shadowy images and shapes that dwell within the mist do not change or get worse, they are the same for everyone.

This means that the spell is not doing physical harm to your body, but instead the images are scaring you mentally (The wisdom damage). It is not a gaseous vapor that you are inhaling every 6 seconds, it is a shadowy illusion spell that has a specific impression meant to mentally scar you. You are either fooled by it or you are not. You don't simply believe that the shadowy shapes and misty vapor is indeed a spell fooling you one second, and six seconds later completely believe they are real. Cloudkill requires a save every round because you are still subjecting your body to the poisons, the illusions on the other hand are figments, they do not physically harm your body.

Fear effects are compounding.

The bottom of the spell description states that "All creatures within the mist must save or take 1d2 points of Wisdom damage and gain the shaken condition." Under the fear description (page 563 cr) it states "In most cases, the character makes a Will saving throw to resist this effect, and a failed roll means that the character is shaken, frightened, or panicked.", it goes on to say that "Fear effects are cumulative, a shaken character who is made shaken again becomes more frightened, and a shaken character who is made frightened becomes panicked." This addresses the "shaken" part of the fear spell.

Final Conclusion

A creature would only make a save. If they fail their save, they are mentally scared and take 1-2 points of wisdom damage on top of being shaken for as long as they remain in the mist. Leaving the mist after failing a save would mean that the character still keeps the wisdom damage, but loses the shaken effect. Should they re-enter it, they would inherit their last fear effect (Shaken, Frightened, Panicked). Fear effects are disbelief based, which means that the second a character makes a successful Will save, they no longer have to make another save, they already know that the spell is just an illusion.

However

It does state that fear spell are compounding, and fear effects go from Shaken to Frightened, Frightened to Panicked. As a DM you could have players make multiple saves should they fail their first one to determine if the level of fear increases, but that is part of the spell is only in place should they already have the shaken condition. The Figment part of the spell suggests that because the spell does not change, the player would not take consecutive wisdom damage, the damage is already dealt upon seeing the shadowy shapes. It is up to you as a DM to decide the final ruling.

I think you're simply reskinning Darkness. Instead of blocking all light, you're creating so much light in an area that you can't look directly at anything within 15" of the target. Reskinning this way still ticks the conditions of the spell itself:

• Darkvision doesn't help, because the area isn't really dark.
• Non-Magical Light doesn't help, because adding light to an area that's already ridiculously bright is superfluous.
• Completely covering the object it's cast on with an opaque object blocks the effect.
• Spells of lower level are dispelled. You could easily say it absorbs them into it's own effect, rather than an traditional cancellation. The end result is the same.

Cinematically, you can say people "must" close their eyes when looking at the area, but the game doesn't have any rules for whether a character's eyes are opened or closed. A character can look into the area of the spell all she wants - eyes open or eyes closed, she still can't see whatever's in the area or beyond it. "Turn" far enough away (read "not interacting with something in the area"), and it's not a problem.

It's worth noting that the 5e Player's Handbook has no rules on facing, but there's an optional facing rule on page 252 of the Dungeon Master's Guide. If you're looking for something with non-finite range that can force facing, you should probably make a new spell from whole cloth.

If you're looking for a persistent effect on the target, then Caleb has the right idea.