Mastering, as music production, is something that usually doesn't happen all of a sudden. You need to put in work. You do have happy accidents, but most of the time it's skill and knowledge.
There's two facets to mastering. There's the software and there's the hardware. A lot of ppl try to short cut there way to a master by only using software and monitor speakers. Ppl that offer professional mastering have the knowledge, skills, treated studio, proper hardware and usually some sort of software. There's a lot of amateurs out there that also offer their services, usually lacking in one or more of those departments. It comes down to what it's worth to you. A lot of producers have their tracks professionally mastered and pay for it. If paying is not a viable option for whatever reason ( in my case, I'm nowhere near earning money with my hobby) , you can see how far you can get yourself.
If the methods don't seem to work for you, it sounds to me your lacking in one or more departments as well. Big chance it being theory and skills. You should be able to brush up on those by the help of the internet.
https://ask.audio/articles/9-essential-cubase-mastering-tips
https://www.youtube.com/watch?v=yDViKqK53IQ
https://www.youtube.com/watch?v=iguKLoQmtVo
https://www.youtube.com/watch?v=aVLX7mT7EgM
Put effort in it to get somewhere I'd say. Find a nice balance between learning/reading/listening to theory and incorporating it. Don't expect to ace everything straight from the start.
How's your hardware setup? What AI, monitor speakers, room treatment?
If this all sounds like a big hassle, you rather concentrate on the creative part and you think it's worth throwing money at it to have it professionally mastered, that's also a nice route to take.