##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.
The Problem
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.
The Solution
A master tier player will throughout every second of the game try to play it as fast as possible (taking the camps as jungler fast as they can, optimizing the roaming 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.)
IMO Tempo is one, if not THE biggest, determinatior of gamecarrypower and raw skill in general. It has a monstrous learning cap and a lot of impact, so it is definitely worth investing dedicated time into it, when trying to climb/getting better.
How to practise Tempo
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.
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.
Answers to generally asked Questions
Q: How does Tempo win me the game?
A: Tempo doesn't. You do. Tempo is the tool, that amplifies your impact in the game. If you are ahead, it will increase your positive impact, but if you are dead behind, it CAN make you feed harder. It is not simple at higher tiers, but straight forward at the beginning.