My advice is play whoever you genuinely enjoy more. A lot of people may not like this advice, but this is a game, and you're supposed to enjoy it as much as possible. There's no real point in playing if you're not having fun.
If I had to pick between the two, I'd say stick with Kat since you've been playing her a lot recently (so you're currently used to her), and try to look up different guides / strategies / ask for tips on how to improve further. If you're getting fed, but struggling to end the game, it sounds like you need to change the way you go about things after that point.
Be more of a shot caller and try to get your team doing things together. It's also a good idea to help / ask for help to push a bit after a successful roam / gank, etc. The goal is to end the game as quickly as possible with Kat imo. She's really strong early/mid game, but falls off later on if she doesn't keep her lead, or is no longer able to blow people up / gets CC'd.