Cry as long as you need to.
I'm not going to try to stop it, or tell you to breathe through it, or ask you what's wrong. Crying isn't a problem to fix. It's your body doing something with all of that.
The only thing I care about is that you stay physically safe while it happens.
While it's happening
Sit or lie down somewhere you can't fall from.
If crying is turning into not being able to breathe — chest tight, hands tingling, feeling like you might pass out — that's usually panic, and it passes. Sit on the floor so you can't fall, and let it peak. If it doesn't ease, or your body feels genuinely wrong, get medical help.