1) Keep your sensitivity at what feels comfortable. If you feel you can't react fast enough to scouts or something fast, raise it. If you're overshooting, lower it. If you're undershooting, raise it. Just don't change it a whole lot. You won't magically have perfect aim after 1 or 2 days of using a new sensitivity.
2) Don't play for ridiculous amounts of time without taking a break. You'll just feel like shit if you play too long, especially if you tense up when you play (don't do that).
3) Find a HUD that YOU like. Just because b4nny or yz50 uses something, doesn't mean you'll do just as well with it.
4) If you're having an "off" day, just focus on what you're doing well with that day. So, if your aim is off done day, practice your movement. If your movement is off another day, practice your aim, etc. That way you make the best out of your time. You always need to be working on something. Eventually this will come together and you'll learn to adapt better. And off days are normal; treat them as an opportunity to improve.
GL