Path of Exile 2: First Principles

Steam Community guide by reth.

Forget what you know

Path of Exile 2 is poorly named. This is a completely different game and experience than Path of Exile (1). Better said, Path of Exile 2 is a step in a different direction than the other "diablo-like" games that defined the Action Role-Playing Game genre.
Action Role-Playing Game is now too broad to describe the kinds of games within it (PoE2 is not the the first to be a cross between souls-like and "diablo-like").
If you are coming from PoE1, forget what you know and intuit. Some macro-level core principles apply (like focusing on defense and the major nodes on the passive tree), but PoE2 is fundamentally different.

Zero money is the correct money

Coming from Real-Time Strategy games, keeping your money at 0 is often good advice. In PoE2 terms, that means it's okay to spend your currency and upgrade materials on upgrading your character.
If a boss feels hard (like can barely get them to half life), then an equipment upgrade can go a long way. Even adding a rune to a weapon can be a 33% damage increase (or more!). Upgrading 2 or 3 pieces of equipment can be the difference between a souls-like experience and a game that feels too easy.
For example, I got stuck on a boss (not even down to 50% of their health and died 3 times) and I haven't spent any of my currency in my first 3-ish hours of playtime.
I spent my next 2 deaths just watching for the big attack that I knew killed me. Seeing the pattern, I knew what to look for and upgraded a few pieces of gear with extra damage and frost buildup. It was enough to take the boss down on my 1st attempt after upgrades with as much ease as taking down a normal pack of mobs.
Obviously, loot is random and player-to-player experience will vary (maybe you're in the lucky 1% that will have received almost no good loot -- yay you!).
While personal skill is a big part of the game (and someone with enough skill can probably kill these bosses with just white items), the itemization is strong enough to carry average players (such as myself).

Points of Notice

I've always been a big fan of having the massive map cover the screen as an overlay. Let me see everything at once, scrolling a little here and there to find fringe locations as I teleported across it.
PoE2 is different. Movement, positioning, and animations are way more important than in "diablo-like" games. Especially since even a normal mob with a strong attack might be able to do significant damage. Awareness of your surroundings is very important. It's not like other games where the screen just dies so you don't have to worry about what you're actually fighting (I'm sure the endgame will have this style of play).
Combat is grounded and knowing the terrain and enemies will affect how a player moves and fights their enemies. All this to say:

• Move your map to a top-corner of the screen
• Keep your eyes on your opponent, not yourself
  The other big component is to have a game plan with your build. For example, I'm playing a monk. I have gems on my default attack to increase attack speed and increase stun rate. I use it for my primary damage, to build stun & freeze gauges, and to interrupt weaker attacks.
  The other half of my build is to build power charges with my teleporting-finisher attack and to use those power charges on a larger area-of-effect attack to clear packs of lesser mobs with (or do a good chunk of solo target damage).
  Given all of this, I have a plan in the back of my mind for how to deal with different enemy compositions before I even see them. Then the only factor I need to worry about is the terrain and positioning.
  Figure out what your plan is and adapt it as you play and learn.

You don't need a build guide

You really don't. If you're starting to notice problems, slow down and think about what you need.
More personal skill is rarely the answer. Things can be solved by re-arranging what you already have, the flexible parts of your build and items, and spending a little currency.
If you're in a good spot, remember your past problem areas -- they are likely to come back so start working towards addressing them now. If you're feeling good, then shore up your position.
Play what looks fun and interesting to you. Even if your intuition is off, then learn something from it and try again. Discovering and solving build problems IS the main source of joy from the broader Action Role-Playing Game genre -- embrace it.
The difficulty you're facing is the game's feedback loop of telling you what you're doing wrong. Use that feedback to learn and improve.
Don't know where to start? Try here: https://steamcommunity.com/sharedfiles/filedetails/?id=3385926466

Useful Links

Basic to Average Guide: The perfect follow-on guide if you need more guidance to get started (no major spoilers)