How to split a combo in osu

It ultimately comes down to memorization. Just like playing music, it's very difficult to get everything on the first try, especially on harder difficulties. You need to play through a song a number of times to let muscle memory set in.

With the relax mod, there's not a way to check what your acc, score or combo is until after you've finished the play. Even when watching the replay, you won't be able to view any of this.

Edit: This is now possible. I think it shows by default now but you can press Shift+Tab to show the HUD now.