Singing, unlike speaking, requires the holding out of notes.
Sometimes the note needs to last longer than you feel you have the breath to support it.
Here are three ways to make your breath last longer than you ever thought possible:

1. Don't leave a breath mark in front: Put the palm of your hand very close to your mouth.
   Now try singing a note out, but letting as little breath as possible get on your hand.
   Or...
   Imagine a mirror right at your mouth.
   Hold a note, but try not to leave a breath mark on the mirror.
   You can use a real mirror for this or just imagine it.
   If you do it well, it will almost feel like you are not exhaling any breath at all.
   
2. Imagine pulling, instead of pushing, sound up through your mask (eyes and nose).
   This will require your eyes to be involved in facial communication, and for your nasal membrane to expand.
   It helps the pulling sensation by "talking" with your hands, also by imagining your most important audience to be above and behind you.
   
3. Do staccato vocal exercises, making sure your belly bounces in and out as you do.
   Do these slowly at first until you can learn to allow your belly to open up in between notes.
   It feels a bit like panting.
   A good pattern is "G-B-D-G-D-B-G-B-D-G-D-B-G" on the alternating syllables "ha-he-ha-he-ha-he-ha-he-ha-he-ha-he-ha", then take the pattern up by half steps until you are quite high in your head voice.
   This teaches breath efficiency.
   Be very sure to allow your jaw to be free and flexible, dropping it more as you go higher.
   

With your breath correctly compressed at the pelvic floor, your eyes active and your jaw relaxed, you should get volume from the rich resonation of sound from your open throat.
No pressured tone would sound as beautiful.