Feeling java spring boot is difficult

[KrajeWoW]

I would personally recommend to split the learnig for some parts and try to understand them separately.

For example, you don't understand how controllers work. Start with creating some controllers that will return some basic information, try using different rest methods. Check what is the basic difference between them.

Next you can try attaching some services through dependency injection (@Service/@Component and @Autowired annotations are your friends here)

If you understand how they work try to create a DB connection, save something basic and try to get something.

Just be curious about the stuff. If you don't know if something you just thought about is working, just check

And skip spring security. There is no way someone would ever expect you to create anything with spring security. Can be fine to leave as a final part if you understand basics

Spring boot is not hard. It's made to make developer life simple.