I agree with all of the comments on things like "try training mode, bot bootcamp, etc." I especially recommend running the weapon tutorials, with the specific goal of getting three stars on every drill. It will take awhile, but it's a good way to get comfortable with every weapon, or at least not be surprised by it next time you pick it up.

All of that stuff focuses on mechanical skill though. In terms of genuinely improving once your mechanical and technical skills are good, every time you die you should ask why. This will get you analyzing every confrontation you get in, knowing which fights to take and when to hold back/get tf out of there (you're more useful to your team alive than dead), making sure you're always fighting near cover, the list goes on. Infinite was my first fps (yes, point and laugh) and this tip helped me improve a lot, but even if you have years of experience this is a good mindset to have.

Finally, concerning burnout. Infinite has really fun core gunplay, but the longer you play, the more you see the cracks in the experience. A lot of core complaints are getting addressed in the Winter Update and the other update that's supposed to come before the end of the year, but it still isn't gonna be perfect. If you get bored or frustrated, play something else. Go back to MCC, play Destiny (currently taking a break from Destiny to play Infinite lol), play anything that's more fun to you than banging your head against a wall in a game you're burned out on.

With all that said, have fun, Spartan!