How do I hold back my tears when I'm in public?

If you are already in public a good way is to breathe deeply a few times while trying to direct your thought to something positive. Try to keep your thought under control and push away the ones that makes you sad as soon as they arise. It is not the easiest exercise, if you manage this you should be very proud of you, especially if it is your first attempt. It is a bit easier if you are still by yourself and you still need to go out in public. Then you could use your favorite playlist, like a "cheer up" playlist of songs that are able to empower you, that makes you happy. At the same time you still do the above exercise, by pushing away difficult thought. This is how you can hold back your tears, but in the end you cannot deny your feelings, they will just grow deeper otherwise, once you have time for yourself again, explore them, do mindfulness exercises, find someone who can listen to you.

A trick I've learned to do was to take deep breaths. Although that seems cliche, I found that it helped stop the constriction in my chest that I felt whenever I was going to cry. Blinking helps, too, but sometimes it just pushes the tears out. Another thing is to focus really hard on something, whether it be a random passerby, a window, or something on your shoe. If you distract yourself from whatever is troubling you at the moment, the less likely you'll feel the urge to break down. Lastly, you can always have and inner mantra! Mine was basically to sing the lyrics in my head of this song I liked to divert my thoughts to the enunciation of the lyrics than the thoughts that unsettled me.