The thing to think about is reusability. Are you copying and pasting code into multiple places? That's a great candidate to become a class. If you have long lived projects (i.e. something you will use multiple times over a lot of years) maintainability is important. Huge functions and monolithic applications are very hard to maintain over time.
Break your functionality out into small chunks (methods and classes). Keep it simple. It may take a while to get used to this, but your time for adding additional functionality will be greatly improved in the long run.
A lot of great programmers were terrible at one time. Don't let your current lack of knowledge of principles stop you from learning. One of the biggest breakthroughs I had as a programmer is changing how I looked at architecting applications. Following SOLID principles will assist a lot in that. Don't try to understand and use these principles all at once, take your time. Programming isn't what you make your living with, it's a tool to help you be more efficient in your current role.
Realize that becoming a more effective programmer is different for everyone. Like you, I was self taught. I was a systems and network engineer that decided to move into software development. I've since moved into a role that takes advantage of all the skills I've learned through the years in SRE. like you, a lot of what I write now is about automation and analysis.
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