In 5th gen I had this thing where I refused to use any mons but eeveelutions. I was losing 3 out of 4 of my matches on battle spot on average. It wasn't until I'd had over 1000 battles that my ratio of wins to losses reached 1:1 again. I've played tonnes of different formats. 3v3, 6v6, doubles, triples ect. Anyway since then I've used other pokemon (except for umbreon I refuse to play without that bastard on my team.) Your best bet to winning with your favourite mons often is relying on the one advantage you have. The element of surprise. After a while of fighting standard smogon sets you'll find that you can pretty much predict almost every move they'll make, but no one expects a swords dance leafeon to one hit ko their ferrothorn/scizor with a natural gift. Yes you may have to rely on some gimmicks like this but it's your best bet when fighting against mons with better movepools, typings and BSTs.
Anyway I've improved since 5th gen. Even winning a VCG tournament with an umbreon on my team. Sadly half my team was pretty standard though. I feel that I've somewhat deviated a bit from my karen fag roots.
http://pokemonaustralia.com/nsw-pokemon-vgc-tournament-april-ggs-town-hall-winners-report/ On the other hand recently my best friend came 4th in the NSW regionals using an entire team of his favourites. Including a shiny furfrou which he's in love with haha
http://pokemonaustralia.com/pokemon-vgc-regionals-sydney-recap-teams-and-usage-stats/ 