Fear and Doubt vs Reality

Interesting how I worked on my two piano songs today and the thought process that may have been going on recently.
As I ended working on song one "These Foolish Things," and continue to work on song two "Somewhere," the thought goes on something like: uh oh. My interest is going to fade because I'm going to forget song one, learn song two, etc. Its almost like I abandon my other child (song 1) in favor of the new baby (song 2). Hat tip to Billy Joel for the analogy of new songs = new born babies. Other thought creep in too, like: the old problems will resurface...I'm going to confuse parts of songs....
But I'm noticing a funny thing. Reality. I can still play song one ok while working on song 2. A couple of possible contributors:
A) I took my time learning song
One until it was learnt to
Play at a DECENT speed.
B) No rush to learn!