I don't want to adjust CPU sliders to be honest, once I get into that whole area I'll end up spending more time mucking around with them than enjoying the game.
That said, I've been fine with it up until this last week. I've only been playing friendlies with random teams, but I've generally done okay - won some, drawn a lot, lost some.
But this past couple of weeks, it's as if the COM has gotten a whole lot more aggressive. It hasn't as there hasn't been a patch, but it feels that way. The game is starting to thrash me almost every time. I just turned it off when I went 4-0 down against a lesser team.
I can barely score at the moment either. Once I get to the final third, all my players seem to just stand around doing nothing, being man-marked. And at the other end, I can't stop the COM from scoring, particularly in the last ten minutes. It feels like the game just scores when it wants and you can't do anything to stop it.
It's a great game, the AI on World Class is amazing, but I'm not enjoying it because I'm so bad at it. I can't even contemplate doing a Career Mode because I'd get sacked.
So I dunno. I did okay at the game at first, thought it's fine, I'll improve the more I play and learn the game, but I'm not improving at it at all. I'm now getting thrashed every other game. I mean I lost 8-0 yesterday, and twice switched the game off tonight when I went 3-0 and 4-0 down respectively. Switching it down to Professional level just doesn't have the same feel to it, so I'm not sure what to do with the game at the moment, because getting hammered all the time isn't much fun.
Well I play with the default settings (except for injuries) and I find the game an absolute joy. I did just yesterday move up to Legendary, so we'll see if I stay at default, but so far so good.
That said, personally I kinda think you have to make slider adjustments if you're going to play with manual settings, especially on the higher difficulties. Unless you're just awesome which, no offense, it doesn't sound like you are.
If you're going to stick with manual, you should really try different slider settings. Personally I prefer Orion's settings over at Operation Sports, but you could obviously try Placebo's too. You could try both, pick which you like the most, and just promise yourself you're not going to tinker with them.
You could also just take a break from playing manual for a while. Moving back to assisted can allow you to focus on other aspects of the game and allow you to keep playing the game while limiting the stress, and it might allow you to better recognize what you've been doing poorly. (I actually did this and I just haven't gone back to manual because I've been having so much fun on the default settings.)
You might also try actually playing on Legendary for a bit. If you think the AI on World Class is amazing, then you'll be blown away by it on Legendary. It's pretty awesome but in some ways the game is a little easier because it forces you to play smarter and more conservative. Playing on Legendary might help reboot your FIFA skills.
Last, taking a break from FIFA when it gets terribly frustrating is always an effective thing to do. Sometimes taking a few days or a week off is the best answer, and then when you come back the game feels so much easier to play.
(Also, there's no shame in starting a CM before you think you're ready. In fact, I would recommend it as it plays different than exhibition. Just do what I did and run a few "test" CMs. In my first, with Spurs on World Class, I got fired within a couple months. In my second with Chelsea, I made it past Christmas before getting fired. In my third, again with Chelsea, I finished in fourth in my first season, won the League in the second, and now starting in the third season I've just moved up to Legendary. So don't stress, you'll get better.)