Casual play and competitive play are very different. Experience in casual play is only applicable to competitive play to a very limited extent; you basically have to relearn the game from scratch when you start playing competitively. Therefore, you shouldn't expect your control of your character to be good just from playing casual games, with no competitive experience and limited focused practice.
You will be physically able to execute faster and more precisely with practice. You will also learn how to apply your options in ways that allow you to be faster and more precise.
Do deliberate and focused practice sessions rather than just moving around mindlessly spamming techskill or comboing CPUs. Actively critically evaluate your play and look for habits, areas for improvement, things to think about, and so on. Watch competitive matches and analyse them; think about what options are available in a situation and the properties/consequences/advantages/disadvantages of each, and which ones the players choose and why (and also the other way around: what situations the players choose to apply certain options in and why).
The Melee Library is a great compilation of useful guides (including many on improvement and practicing):
https://docs.google.com/document/d/14scMKnw-IyD_FhOy6Pg7EAkRwu-e1YNbA06k0pwZpF0/edit?pli=1
Control of your character is extremely important, because it's necessary for applying every other aspect of the game. Learn your basic techskill, and make it consistent. Once you can do what you want when you want it, then you can start to focus more on your decision making.