The reason it is inconsistent is because those songs hover around d4 to a4. brightside if you play starting on D major. Sings d4 the whole verse then goes up to some f# and g's so if you sing constantly on d4 you could get very tired by the time you go up to f# you are gonna feel you have to push. This is where perfect practice (vowels, breath etc) is gonna be really important. Knowing exactly how to approach. If you just keep blindly throwing yourself at it it's gonna be a long hard road. Basically think of it like you are lifting a moderate heavy weight but a ton of times. It's gonna get tiring.