Wecome to the club tilt.

I've been in the exact same place time & time again myself. And I'm a trainer, I TEACH gunnery!
Part of what helps me is when I get to where I'm having trouble hitting I go back & practise.
Yes offline vs drones, TA vs new guys, anything and anywhere but the main where the pressure is on.
Connect and computer speed do have a large inpact. You may want to think about an upgrade esp in the CPU.
Also I'd say tex is right on the money.
Saddle up, get him to burn his E, wait for a good shot, but preferably one that has him showing more than just the knife edge.
Get close, then get closer yet.
A shot thats very difficult at 350 gets much easier at 150, but you can get too close also. If your inside the 100 yard distance any moves he makes makes it harder to stay in sync with him.
Last, concentratrating too much on saving ammo always screws me up.
I end up without both the ammo and the kill.
When it gets really bad for me I start flying the Yak9T. I have 32 rounds if I use them wisely I can get 3 kill sorties in it vs spits & nik's. (works wonders on bombers also)
But it forces me to make every single shell count. It has to be the best shot I can get or its more than likely wasted.
If there is anything I can do to help give me a yell.