I feel your pain...in my office I'm youngest (until recently), one of only a small handfull of women, and don't report directly to the groups I support. I have no choice but hold it together.
It can be tough, but you know what...you have to let it out. We all have that fine line and once crossed, you have to let it out. Better to cry than throw a full out tantrum (which I've seen the guys do from time to time around here).
You don't have to always be "together". It's okay to let yourself be human. Just think...the world didn't end because you cried at your desk.
What I've found is that I need to get sucked into something else when I get stressed. For example, I will walk to another location in the building, or read something quick on my cell phone, or heck, come to this site.
Once you find your happy place/activity...keep falling back on that. Just know, that the world won't end if you're human and experience human emotions.