Teaching is hard, kids are frustrating, adults are bad people, and you are being expected to do way more than is reasonable. Now that all of that is out of the way, let's talk about how to cry.
Don't cry in front of the kids.
There was a teacher my first year, a 10+ year veteran, who could be heard loudly weeping and yelling at kids depending on the time of day. Not only did it make the kids behave worse, but it disturbed and terrorized the nearby teachers as well. If you need to excuse yourself because you let a child get under your skin so far that you are about to cry in class, do it. Take the write up. Better that than crippling yourself for the rest of the year. No, it doesn't look good, but i'd argue the kids seeing the tears is worse. Kids can be like piranha. Once they smell blood in the water, you're done.
Let it out regularly
It doesn't have to be tears, but relieve that stress somehow before it gets to the point where crying is mandatory and involuntary. If you let it build up you're going to explode on a student that probably is doing something on a much lower level than you are percieving it to be.
Reset every day
Don't let yesterday determine today. You start fresh and let the students, admins, and everyone else start fresh too. There is only one student I've ever had that I actually don't like, but sometimes I let all that negative emotions stack up and bleed into my relationships with other students. At the peak of my feud with this one students, she told me she didn't like me. I told her I didn't like her either. I'm serious. I still don't like this kid. How crazy is that? I wouldn't have said that if I hadn't taken her constant defiance and under performance personally. I should have let her start over, instead, I decided early that she was terrible, and honestly, I still feel that way. I was in a year long emotional feud with a 15 year old girl. Who's fault is that? Mine of course. So dumb.
Regardless of the reason, if you need to cry, do it. Find someone you care about and let it out. You'll feel better and you'll be better.
New Teachers are allowed to cry - NPR
DISD Teacher Tears Joy
DISD Teacher Tears Overwhelmed
DISD Teacher Tears Hopelessness
DISD Teacher Tears Frustration