Feedback on first PoE character

Hi all,

I am new to Path of Exile and I've gotten to level 57 so far with little trouble. I am currently in act 6, but I am noticing it's getting harder. I think the main reason is my lack of resistances, but I also thought the Witch would be a bit of a glass cannon.

I clear mobs incredibly fast with ice nova, bosses take a tad bit longer, but not super long. Act bosses seem to be a bit harder and the end of act 5 wasn't easy at all and I died multiple times, but I suppose it wasn't supposed to be easy.

I am just curious to see if you guys have any feedback for me that could help me progress further and maybe I'm missing something totally obvious! I've been a long-time Diablo player (1 through 4), didn't like 4 and found out about PoE, which is amazing so far. I don't like to follow build guides and prefer to discover things myself. This is how I ended up with the build I am using now.

I look forward to hearing your thoughts about my character (I only have 1).

Thanks in advance!

Completely normal for your first character. Priority over anything, fix your resistances, damage doesnt matter, if you are alive you will kill it.

Pretty normal.
The game doesn't do a great job of teaching you defenses, so this is quite normal. In Softcore, it doesn't really matter if you die occasionally, but dying repeatedly to overcome something is usually a sign you need to think about defenses, and once you're past the campaign and into maps, you will usually want to make sure you don't die too often - you need a REALLY big cannon to be able to be a glass cannon.
Generally, you want to layer "defenses" a bit. Without a complicated gimmick build, you need Resists and one of Life or Energy Shield, plus a few from the longer list of secondary defenses - Block, Spell Suppression, Regen, Leech, Ward, Evasion, Armor, Endurance Charges, and some Skills. Golems can help a lot - Stone gives a bunch of Health Regen and distracts some enemies, Chaos Golem gives Physical DR. You don't have to go TOO into a secondary defenses, but having them is a great benefit.
I hope that's helpful for planning!

You want overlapping defensive layers.
Capping ele resists is the first step to this defense. It is the baseline for how the developers tune the difficulty of the game. They expect you to have 75% ele res by act 4-5. The first mobs in act 6 are fire mobs that expose fire weakness so they will chunk you if you aren't fire capped.

You can go to your crafting bench in your hideout and put a singular resist on an open suffix for very cheap and quick fix. There are a lot of 1chaos uniques on the trade site that also offer some reliable protection.

The second layer is the armor/evasion/energy shield layer.
This isn't as important, but it does have some effect.

Third layer is avoidance. Just avoid the attacks. Either by killing the mobs before they show up on the screen or by moving around them.

Fourth layer FLASKS, and this is surprisingly one of the most important defensive layers. You will want at least a single life flask that removes corrupted blood. Quiksilver is also a good defensive option because it can help you move out of the way of enemy hits. The breadth of flask uses is too wide and ranged to ramble on here, but I highly recommend using flasks. A good way to play is pounding on the flask keys and then automate them once you get enough instilling orbs to get the flask enchants.

Some simple breakpoints that I try to keep up with is 300hp per act I would give you damage breakpoints, but the in game tooltip isn't reliable. IF you have ES you can add that ES to your HP pool so if you have 1700 hp and 1000 es you can safely say you have roughly 2700 hp. so you will want 75% lit/cold/fire res and 1800hp by act 6, 2100hp a7, 2400hp a8, 2700hp a9, 3khp a10, and you want to be around 3.5k by white maps 4.5k+ by red maps.

Your ascendancies are almost always a defensive layer. Especially Witch who has 2 overt defensive Ascendancies and one offensive Ascendancy with a lot of defense to it too.

also note that after beating Kitava you take a 30% hit to your resistances, which can also be the reason you feel mega squish in act 6.