Why do I sing flat, and how do I stop?
Singing flat, slightly under the note, usually comes from one of three things: not hearing the target clearly before you sing it, running short of breath at the end of a phrase, or a tired, under-energised sound. What helps is hearing the note first and humming it before you add words, arriving on the note rather than sliding up into it, and keeping your breath going to the end of the line. Then measure instead of guessing, because a few cents flat is very hard to hear from inside your own head.