First, if you're having a hard time with the game, check to make sure you didn't join the Company of Champions.
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When approaching the game's difficulty, there are a few things to keep in mind.
1) This was voted the easiest of the Souls games by GAF (though I disagree with the assessment),
2) Going down and saying "It's harder because of these design choices" misses all of the things that benefit the player. There are a bunch of things that ultimately help the player out, like:
- There are no worthless stats like there are in the other three games.
- If you don't like your spec, or you misplaced a few points, you can respect.
- Crafting comes earlier and at lower cost, and the mechanics are simplified.
- You can upgrade armor.
- There is a far greater variety of weapons and armor to fit your playstyle and current stat build at any time.
- The game is wildly less linear, and even less linear in the updated version due to a few key item placements/purchases.
- Fast travel lets you explore with less effort, and allows you to look for alternatives if you're having difficulty with any area or boss.
- There's overall much greater build variety and balance.
- It is much easier to do meaningful hybrid builds rather than stacking a single stat-or-two.
- You get four ring slots, and there are a ton of options that effectively "disable" some of the things people dislike about the game design. Durability, soul loss, "humanity"/HP loss, amount of HP loss, too-little poise, too-little poise damage, etc. are all things that, if you're struggling with, have solutions in the early game. Keep exploring!
If you're having difficulty, or think an area sucks, it is so easy to reconfigure your character or try going somewhere else. I found Dark Souls 1 to be frustrating until I figured out what spells/gear were broken as hell, but I felt like this game really allowed me to find my own alternatives and progress at my own pace and interests, and that the solutions weren't just "cheese the fuck out of the game". If you go in expecting the pace or mechanics of Dark Souls 1, it will probably be a miserable experience.
Overall, it is about the same difficulty as Demon's Souls, Dark Souls, and Bloodborne, but you have to use mechanics and design to your advantage rather than assuming the mechanics copy over from one of the to other games. You could get away with it going from DeS to Dark Souls, but between "DS"/BB/DkS2, you have to account for the different mechanics.