Motivation is key. When and if you are motivated enough, focus comes as a side effect of it (personal experience).
Having prioritised list helps to deal with multiple projects. Here, it is important not to fall into switching to a lower priority project if struggling with the other one, although context switch also advantageous when not obused.
Limiting number of WIP projects was a delight to me. When one finished, feel free to switch to another one. Different time and place for different projects also another technique to be aware of.
Also through dock you can configure a window to be on all of the screens
*<-using macos 10.14