Something on the level of Murasama is a bit too demanding for one person to manage - especially if they doubt their ability. The programming might not kill you, but the artistic resources certainly will!
To give you an example, its taken me about fourteen years to learn multiple languages, get qualifications, master 3d/2d packages, animation and drawing and its only just now that I feel confident enough to take on projects on the level of say, Myst, Lands Of Lore, Dune 2000...at a stretch the original Buldur's gate. In other words: commerical games from the 1990s...
My advice is to focus on strengthening your knowledge of Java and skills in 2D image packages(GIMP or Artrage). I also strongly recommend putting some time aside learning software development and maths up to algebra level. Your language and maths skills you can teach yourself, but I feel its worth getting formal education in software development as you are being put to the test for completing a program that is not only complete, but also stable and maintainable. With that vital skill, games development becomes a lot clearer and manageable.
For what its worth, I too am a Java programmer working on a game that I hope to release on the Android market. It'll be on the level of Myst and Jurassic Park(Sega CD), so I feel its a reasonable goal. And thats what you should do: work towards reasonable goals.
As for C++ being faster than Java...for games like Crysis then yes, you should be using C++, but otherwise Java is perfectly fine.