Another principle of good singing is to avoid straining to reach high or low notes, particularly when your voice is getting tired. If you begin learning a musical instrument, it takes time to train the muscles involved in playing that particular instrument.
It is the same with singing. The vocal cords are in effect muscles inside the larynx. With practice, your range will increase and you will find it easier to sing for longer periods. The secret of reaching high and low notes is to imagine them before you sing them. Try to develop the ability to hear the music in your mind before you sing it. This visualizing or pre-hearing will help you avoid reaching uncertainly for notes and will increase your general singing confidence as well as your ability.