I haven't played since February (outside of clubs) due to work commitments and also from getting bored of the game, but I hit elite 1 twice in my last two weeks of playing. Despite having played fifa since 98 this has been really the only year where I've played Ultimate team properly (and I still have a lot of problems with the mode), so there was a lot for me to learn to get up to speed (especially the defensive side of the game). Do you know what your weakness and strengths are? After you go on the win streak what causes you to lose, struggling to score or conceding easily etc, or things like game management and holding on to leads? I find that certain formations trip me up more than others and while it's pretty sweaty after every WL game if I was taking it seriously that weekend in the latter stages of the competition I would write down the formation I came up against and anything noteworthy with how I struggled. Sometimes I'd notice a pattern and could take steps to rectify it. I'd also write down things like pass accuracy etc
Tactics and formations are important. It's important to know what the meta is but it's counterproductive to blindly follow it, especially without tailoring it towards your own strengths. At the start of this Fifa everyone recommended either a 4231 or a 442 with lowered depth, with the same instructions. I dislike both formations so I went with a 41212 with attacking fullbacks, but moved to a 3421 with 10 depth, and move back to 7 after a certain amount of wins. I'll often switch formations ingame as well depending on how things are going and how they're playing. One of the guys I play clubs with is a competition player, and when I did play I'd often steal tactics and formations from him, as he enjoyed creating and employing unusual approaches.
I don't know how your connection is but it's vitally important. Because of where I live I use mobile data to play, so if I ever wanted to have a serious crack at WL I would do so at my girlfriends due to her internet. Made a world of difference. Hours on the game are important as well, when I was younger I'd play a lot of different video games but for the last few years it's only been clubs, and this year UT.
The team is quite important, I got quite lucky in that I packed Eusebio fairly early on. I had a pretty poor team as I hadn't spent any money and my reward pack luck had been pretty crap up until that point, but I didn't invest the coins I made particularly well. And just before I quit I ended up creating a weird Spanish only side. I doubt that I'll want or even have the opportunity to go back to playing UT, so if anyone knows a way of gifting another player cards without them being sniped let me know and one of yous can have them. There's nothing that special but there's a couple like Torres, Raul, Portu etc that might be useful for someone with a shit team.