Did you make a habit of always getting armor and a good weapon, and then decide where to go (not just keep running at any target with MG) which was the last thing we told you?
Also, just try to be aware of your surroundings as much as you possibly can, by looking around a lot too. If you can't look around 180 degrees without messing up your movement (when you have to pick up the mouse to put it way back again), your sensitivity is probably too low.
Anyway, if you're getting your ass whooped, that's probably a good thing because you're playing with better players then. That makes it necessary to watch out for every shot an opponent takes at you (good dodging practice).
The most important thing to achieve is that you're always ready and able to look about a second ahead (sometimes less, sometimes more) at movements of enemies. The only way you can hit a dodging player reliably with a rocket, is to predict his moves. Some players make wide dodging moves and some make small ones; if you see someone starting a dodge in one direction, fire a rocket at the place where you think he will move to.
It sounds simple, but most of the time you'll be dodging incoming fire, so there's usually not much time left to analyze and predict moves of enemies. What you gain by practicing is the ability to recognize movement patterns and to make decision much faster, and the fast reflexes to respond to it with accuracy.
It also helps to learn these things by doing them more slowly and consciously. Millions of times people will miss shots because they aim and shoot instinctively at the place where they saw an enemy. The reason most of those shots miss, is because once the shot is fired, the time to carry out the moving of the mouse has taken so much time that the target has already shifted position. With more experience you can do those flick shots (quick turns of the mouse, usually in a straight horizontal/vertical, diagonal line or sometimes an arc), when you're fully alert and prepared to shoot any target.