While behaving in a "grown-up, reasonable, emotionally stable manner" may indeed seem to be "more socially acceptable", I think that that's actually only true in the short-term. When you "sigh, and I ratchet the tension in my shoulders up another notch as I pull my fingernails out of my palm, and I tell myself that I'm fine", these are not invisible actions. I certainly notice them, and I presume that people who are closer to you notice them even more.

Perhaps you should consider actually throwing the tantrums you want to throw. They are an honest part of you. By bottling them up, you become Not-Becky. Not-Becky is a lot tenser and grouchier than Becky. And, frankly, much less fun to be around. If you stay Not-Becky for too long at a stretch, you may find that that is, in the long run, far more socially unacceptable. Your friends can forgive the occasional tantrum. I'm not sure that they can forgive you turning into a betrayal of yourself.