meditation on mantra

i use a mantra in meditation due to trouble breathing out my nose i find it easy to focus on a mantra while breathing with my mouth, but i have read that maybe i should be using the method of watching thoughts instead and having a japa practice seperate for "meditation/practice of concentration" any where where it talks about spiritual practices its seperates meditation and japa im just a little confused that im doing the right thing. any advice would be much appreciated