Do you know what the beauty is in doing your own songs?
The only one who knows how they truly go is you!

Maybe try taping yourself and listen to where you go wrong. Get some clues. Or, have someone else sing it and sing along. When I'm learning a new song, my rule is listen for 5 times and then try it. Maybe you need to just listen a bit and it will come. It will be different than reading it.

Maybe change the song up a little that you place some space in there for you to think?

Not sure if I have helped.