How do I improve?

I've tried on and off since launch to like this game, I really have. But it seems like every time I play I just get stomped to oblivion. I don't understand anything about this game. I've been playing COD since BO2, and BO3 is one of my favorites games of all time, but I know this game is different. What am I missing? I've tried using stimshot, armor, etc. but I feel like I never have a chance. Every single game, without fail, always ends with the enemy team getting strike teams, sniper's nests, and threshers. I try to play tactically by checking corners and not rushing all the time, but I still find myself constantly getting killed before I can kill them. I always get 4-5 shots on them but I always die. Is there a way to get more kills that I'm just oblivious to? 90% of my eliminations are just assists and I really don't know how to get any better. I'm probably average at COD skill-wise, but I didn't think I was this bad. I have the vmp, but good guns do not equal skill. Any tips for getting better at the game? I really want to like this game, but I just can't stand playing for more than 5 minutes because of how hilariously bad I am.