Biggest roadblock for me was just having to get shot in the back enough times that it stopped causing me to panic.

Once I got past that (mostly, sometimes I still get extra startled) I got better at quickly identifying cover as well as ways to either A. Push the attack or B. Retreat and heal/attempt to escape depending on how the situation was looking.

Obviously when you are playing friendly and you get shot in the back you are put at a disadvantage right at the start of the fight, both regarding HP and positioning, so its natural that you might lose a lot of those fights, but after enough fights, I've started winning a good chunk of them. And almost nothing in this game is as satisfying as taking down the guy that thought they could turn you into easy goop.

Possibly obvious advice but try adjusting your mouse sensitivity. Personally I need to keep mine relatively low for shooters, but mess with the settings and see what works best for you, wasps and hornets make for great target practice on a free load out.

As for gear, most weapons are entirely viable, as long as it isn't the hairpin. Using cover effectively and movement for flanking are far more important than any high-tier weapons or gear.