Menuchas Hanefesh is very important in order to have a good davening. Real menuchas hanefesh means living in the moment, not thinking about what you just did or what you’re about to do. Just think: right now, I’m here! Right before Hashem!
It also means that when you wake up halfway thru shemona esrai and you realized that you didn’t concentrate on 1 word yet, you don’t start eating yourself up over it. You right away shift to the present, to what is NOW! And that is your conversation with the most powerful Being that exists in whose hands every human being is a pawn. Good luck… and don’t forget to ask Hashem to help you work on your Tefillah!