IMO the best way to practise and improve your mixing process is to mix other people’s music, especially styles that are new or uncomfortable for you.
“Where can I find Multitracks to practise mixing with?”
Ask your musician friends, the community, Groups like my one, Reddit, or any other way in which you can reach out. We all begin somewhere. Being straight-up with people and simply explaining that you are just looking to practise and broaden your horizons is not a thing any people will have a problem with. No one’s born an expert.
Here is another fantastic resource for downloading free multitracks in a vast array of styles. I would suggest that you try mixing music that is well outside of your comfort zone. This will force you into practices that are new to you and help break your own process. This is essential to your growth.
I’m doing this now with a jazz mix, taking it slow, and brushing up my Pro Tools skills. As this is not a client project with a deadline, I can take my time with it and have fun! Also, i’ve never mixed jazz before, so I feel like a beginner when mixing this stuff. It’s great!
The benefit of practising with other’s music is that you don’t have a sentimental attachment to the music, therefore giving you the freedom to mix the music objectively.