First of all, people telling you to change sensitivity, it should not be your first concern(if you used the sensitivity you have for a considerable amount of time); sensitivity is subjective, only you know what feels best for yourself. Find a sensitivity you feel comfortable with and stick with it for atleast 40-60 hours of playtime. Next figure out whether you are a wrist or forearm player. (With wrist I can help you out most because I'm a wrist player myself). Incase you are a wrist player, when possible, setup your chairs arm rest in line with the desk so that you have a good linear arm position. (This will greatly help with stability). Aiming in ET for the most part requires your strafe tracking skills. Also a good rule of thumb for figuring out whether you are wrist or forearm player: If you can play with a sensitivity high enough that when you flick only your wrist wrist and you turn atleast 180° consider yourself a wrist player. When its considerably less than that (lower sens) you can consider yourself a forearm player. But as you say, it requires a bit of diagnostic work on your own part. Constant analyzation of your errors during your practice times will furthermore help you improve at a faster pace. Good aim isn't build overnight, >>>repetition is key<<< as with a lot of things.
-Third party training games-
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I recommend using an aim trainer such as kovaak 2.0 on steam (9,99€). There are alternatives such as Aimlab(FREE) or Aimbeast(4,99€) but the latter ones have slight input lag and less customizable options. These games are intended to give you an overview for your performance allowing you to check your mistakes and improve on them. Although these games may not always seem fun to play and you look to improve, try to spend atleast 30 min to an hour on them before getting into your game of choice. A lot of aiming is muscle memory, thats why you have to trick and stick with a certain DPI + sensitivity.
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There are a handful of CVARS in et that can help with your visibility for when you get shot such as bloodflash etc (if you want you can message me about these because I don't know them out of my head right at this moment lol).
Like other's have said aim for the head, in 1v1 strafe fights always track the enemy with your mouse and strafe at the same time. >>>ALWAYS<<< try to keep your mouse at headheight of the enemy, have awareness of small platforms, slanted edges at the wall etc, anything that may impact the Y axis. There is such a thing as positional advantage, for ex. running up the stairs and there's an enemy holding you ontop. That is a disadventaeous position for you because when you get shot while going up the stairs, the knockback will push you down the stairs giving you a slight airtime, decreasing your weapons accuracy. There's a lot more to talk about positioning but seeing as this is just a question about aim let's keep it at that for now.
If you have any questions feel free to message me or ask them here, I'd be happy to help you out.
(Sorry if I am jumping topics all over the place here I've never really typed out something quite like this so this is new to me, I know I have probably skipped a lot of information. Point out to me what I didn't mention and I'll answer where or how I can)
The general point I'm trying to get at here is that you should really look into an aimtrainer as these programs will help you improve the fastest way possible.
Source/credibility: 1800+ Hours into Kovaak and counting! 😛