What you should focus on is your Tempo.
Imagine Tempo as having the need to generate as much impact as possible throughout the entire time.
League is not only about making right decisions, but also about making them as fast as you can. Your priority schedule is clean enough, however, if you find yourself still having a small impact on the outcome of the match, the execution speed will most likely be the issue.
Especially applicable is Tempo when playing early-lategame champs such as Shaco and most other assassins, or in your case Morgana. As your strength decreases, you goal is to close the game ASAP, meaning that you want to execute your schedule as fast as possible.
Most lower tier players for example usually don't increase the pace, once they got an advantage. Quite the opposite, they usually relax down, thinking they already got the lead they need to win the game later anyway, which is why most assassins have such terrible winrates on lower tiers.
A master tier player will throughout every second of the game try to play it as fast as possible (taking the camps as fast as you can, optimizing the routes to not waste any time, make fast decisions for the sake of speed, just to not lose Tempo, e.g. backing off of a gank as soon as an advantage is unprobable, just to not waste time, where you could instead take scuttler etc. )
When practising Tempo, a good start is to keep track of your gold. Always try to beat your @5 @10 and @20 minute marks, you will figure out where you can optimize your gameplay, if you just follow this tracking technique. Later on, you will have to integrate map control through objectives aswell, but that will be much more easy, since you understand the concept of tempo and how to estimate value each different decision has.
IMO Tempo is one, if not THE biggest, determinatior of gamecarrypower and raw skill in general. It has a montrous learning cap and a lot of impact, so it is definitely worth investing dedicated tine into it, when trying to climb/getting better.
If you start to understand this "haste" mindset of Tempo, I recommend to watch professional midlaners playing soloQ, focussing on how they play their own Tempo.