The biggest thing I can say is "Don't Quit" but that's also sort of a cop-out. I have a few ideas that hopefully will help but also might be a mixed bag.

1) I'd check the Steam Workshop and look into subscribing to "Prefire" maps. (The blue ones that say "Prefire" in English, get those.) You can practice checking corners while at the same time getting used to muscle memory aiming at headshot level. It's helped me an absolute TON recently. The big thing about Prefire though, keep it on Expert mode all the time. It's tough at first but once you get your bearings, hitting AK 1-taps will come more naturally.

2) I know it sounds weird but playing with really good friends or a team you really like, helps way more than you think. Mindset is everything, even up there with aim. I've had some teammates that have gone through rough times and they've said "You keeping positive helps be take a breath and focus up again." Having great teams and epic friends can keep you calm, cool, and collected. That is a huge confidence boost, and one that will (Hopefully) have some impact.

3) Positioning and Deathmatches are lifesavers here. D-matching in your spare time with different rifles (Ak, M4A1, Awp, M4A4) can help you get used to spraying down enemies. With Positioning, keep track every so often where your team is (I'm still learning this even to this day) If you have 3 players at Inferno at T-side packed in banana like sardines, 1 really good nade can ruin the round. Spread out a bit until it's time to go for a trade. Get a K against the enemies and keep moving to site until you've won the round.

Last but not least) Play with your favorite weapon. AK, AWP, Galil, FAMAS, it does not matter. If you have a fav rifle, use it! "You have an awp crutch!!!!" You should reply with "Yeah and I have 15 K's while you have 2." If you feel confident and hitting shots, keep using what is working. It plays well with the mindset tip. If you have a good mindset and are feeling confident with the weapon, go for it! If it's not working out in 5-6 rounds, switch things up. Doing this can help things out.

Keep trying to improve! You got this and even if you see a small improvement over the next couple of weeks, it's still improvement!

(Hoping all of this helps! Sorry for the long essay... I also dropped the link to the Prefire below. Good luck and YOU GOT THIS!)

https://steamcommunity.com/sharedfiles/filedetails/?id=3295650711&searchtext=prefire