“how do you nurture that spark of desire to learn seriously for so many hours a day?”
If you place yourself in the right environment, you will genuinely enjoy it more and more the longer you do it.
“If I want to do the same, do I need a really good rav or chavrusah, or can one improve simply by learning alone and with onl average chavrusahs?”
I cannot emphasize enough how important it is to “click” with your teachers and chavrusas. I have had some awkward situations with chavrusas and honestly, if it isn’t a good fit you’ll just find yourself disliking the whole experience and that will move you backward. Lechatchila, absolutely, find yourself a rav and/or chavrusa who works well with you. B’dieved, learn alone (but make sure you at least have someone you can go to when you get stuck).
“how do you get proficient in learning rishonim and acharonim- does practice make perfect?”
Yep. No special tricks. Just sit with a dictionary and make your way through it.
“Above all, if I really want to learn seriously, how do get to a level where I can do that confidently and it is fun too (without asking for a rebbi to go thru it with u)?”
It will come with time. I would suggest including some challenging material in your learning as well as some material that you are familiar with or that you find easier, so you don’t feel like you are breaking your head the whole time. The more you succeed with the easier stuff, the more you will feel empowered to tackle the harder stuff.