If I'm having a super shitty game like 1-10 and I feel the enemy's are either being cheapish (like everyone is noob tubing - nothing against it usually but if a whole team does it... no thanks) or maybe they're all on their prestige mode super awesome whereas I'm only lvl 41 so there's a big gap in map memorization and when you don't know every little nook and cranny, ugh. So yea then it's worth it to just switch games and hope you're with people closer to you calibre. On the other hand, if I'm just getting unlucky then I know I can do better so I just push through and hope the next map is more in my favour.
Campaign wise, just lower the difficulty. I initially beat the game on normal, then started doing it on Hard and only got halfway, and it was tough. Fast forward a few months I just started playing MW2 a lot again recently and managed to finish on Veteran after about a week of trying. It was very tough, many checkpoints I had to redo at least 10-15 times. Like in the goulag shower room. Or the estate level, both guarding the computer stuff and also after that, the escape. I actually raged at one point and punched my couch armrest pretty hard... it kinda hurt since I hit the wood under. But, I calmed down a bit and eventually pushed through.