Is there a better way to describe the difficulty of doing two things at once than to use the example of rubbing one’s belly and patting one’s head at the same time?
As difficult as that can be – now imagine if you didn’t even know how to either rub your belly, or pat your head.
This is what learning a new song can be like: you are learning new chords, rhythms, AND trying to sing – all at the same time. No wonder you are having trouble!
Here’s my suggestion: take a look at the song. Look for odd chords you aren’t familiar with and spend a little time learning how to change to them from the previous chord, and from them to the one that follows.
Then try playing through the song without singing – just sort of hum the tune, or not sing at all and get the music to sound comfortable.
Then do the same with the lyrics. Sing it without the music, or along with a recording, so that you get familiar with the rhythm of the words.
Now, after you are good at doing both of those separately – try them together. Slowly. You might even choose to just hum the lyrics at first, so that you can concentrate on the chords and rhythm.
Allow yourself to go over it a few times – at first somewhat loosely and rough, and then with each passing attempt – refine the areas that need work.
Trying to do it perfectly the first time may be fun to try, but to really get it in your head – have patience and do it step by step.
And by the way, – can you rub your belly and pat your head at the same time?