I also have the same problem

Actually, up to level 20 is incredibly easy for me and then after that I get my butt kicked. It doesn't help that I tend to do stupid things because they seem cool (for example run an Arcane blade with no armour, and only two handed techniques -- no spells except for aegis tree

). But even on the rare occasions where I'm being serious, I get to the mid twenties and then it all starts to fall apart for me.
I don't know what other people think of this, but I have been watching David Fuxa's "Let's Learn" series recently and realized a lot of things I was doing stupidly. Here's a link to the You tube playlist if you want to watch it:
https://www.youtube.com/watch?v=sqZfski ... lNSC-52aZk It is really, really, really long, so if you go to the quality settings in the bottom right of the window (and are using a modern browser that support HTML5) you can set the playback speed to 2x. This may allow you to get through them in your lifetime.
As I said, I realized I was doing a lot of stupid little things. They are all things people have mentioned on this forum before, but I didn't actually understand exactly what I was doing wrong until I saw it in action.
1) Don't rush out into areas where you don't know what's there. The "Let's Learn" series doesn't really make good use of telepathy type items/skills, but getting a quick wand to map everything out in advance completely changed things for me. Especially with a quick wand and some charm skills you can get the cooldown to 4 or less turns, there is practically no downside to using it.
2) Related to 1, wait for the enemies to come to you unless they have ranged attacks or you are worried about them using some ability. This usually allows you to get the first hit, which is incredibly important.
3) Inspect *every* rare. Inspect *every* monster that you are not intimately familiar with. Look at their talents. Read the descriptions carefully. Count the number of talents that you are worried about.
4) Incapacitate any monster that you are worried about. Stupid, I know, but I played for an embarrassingly long time before I realized that stun puts 3 talents on cooldown. So if your enemy has only 3 talents, you only need to stun them once to shut them down completely. Stun them twice if you want to shut down more. Obviously how you incapacitate depends a lot on what character you are running.
5) For fighting against long range types, show yourself for one turn and then hide. Wait for them to find you and then incapacitate them as quickly as possible. There are exceptions to this (like where you need to nullify an important ally/skill as quickly as possible), but that is the basics.
6) You have at least 3 inscriptions. You have one charm slot that uses an item with a global cooldown. You may have other items that have other cooldowns. If you are fighting someone strong and you haven't used as many of these as possible, you really limiting your abilities. If you wait because you "might need it later", it will almost certainly be too late. Use your stuff as soon as you have any inkling that you might need it.
7) Teleport/phase door is not your only means of escape. It might not even be your best means of escape. Have many, many means of escape (movement inscriptions, movement/global speed talents, jumping/tumbling talents). Use them liberally.

Rest completely after every encounter. I seriously hate this one. I don't know why. But it is stupid to wander around with half your stamina whatever, or while injured, or while important talents are on cooldown. One small oversight will mean a nasty rare/boss will walk up and crush you.
9) Kill one and run away. Rinse and repeat. You don't have to kill everybody in one go. This is similar to point 8. It is crazy to think, "Oh, I can just manage to get through these 10 guys. I will be pretty beat up at the end, but I can do it!". After the 10 guys are done, a rare walks up and has you for lunch.
10) Run away and *never* come back again if the situation is infeasible. Obviously there are some required quests, so you have to break through somehow. But there are times when it just isn't worth it. You don't have to kill every boss in the game.
Anyway, those are my top 10 things to work on. I think the problem for me (and possibly for you too) is that the game scales against your level. So you can go into Lake Nur at level 15 and pretty much have an easy time of it as long as you have basic tactical skills. If you go in at level 24, then you are going to be inundated with crazy strong enemies. At lower levels, your character only needs to bash things around and be reasonably careful not to get surrounded. But as your character gains skills and abilities (and equipment), so do the rest of the monsters. If you can't find a way to shut down their abilities while at the same time making the best of your own, then the game just gets harder, and harder and harder as you get more "powerful". You need to be more and more aware of the effects of all the abilities and less worried about "how can I max my damage/defence".
Or at least that's my theory (yet to be tested -- I still suck!!!)