A lot of time on pop sheet music, what they sell on sheet music is not what is on the record/movie/CD/track. This may not be your problem. Pros use fake books, which violate copywrite but mostly have more correct version. You can't buy these in stores, only at musician's union parking lots out of the trunks of cars of the bootlegger.
If you could find a teacher, he/she might keep you from injuring yourself with incorrect posture. Also, he or she could help you with this. Ask around a church or elementary school. There are music store pros, but the teachers that work out of their house are usually cheaper and often are better.
The kind of questions you are asking are part of "music theory" if you want to buy a book on it.
As far as going slowly, that is what you do until your lower brain has memorized the movements. You don't do music with your frontal cortex, that is what you learn with, but is not what you play the songs with. You go slowly enough that you make no mistakes, then when that goal is achieved, you can speed up.