I am right there with you. I am very introverted, while still needing the occasional interaction with folks that I feel VERY comfortable with...usually family and a small handful of others. Interaction is fatiguing and stressful...mentally and emotionally Fatiguing enough that it can carry over into the physical. I have learned to deal with it, over time. So, not quite at the bullet swallowing spot anymore.
In the gym, to the points above, I keep my earbuds in, And while I don't have "resting bitch face", my wife says that my normal look is one of intensity and seriousness. That, undoubtedly, carries over and is magnified in the gym, as I am very focused on what I am doing. That keeps 99% of the folks away. In fact, the only folks I ever converse with are the ones I had to ask for a spot, and the most extroverted guy in the gym. He is a great guy, which helps, but even he rarely comes up for idle chatter.
I, for one, don't want to seem unapproachable, but would rather be the unsociable old guy than have my workout focus disjointed by interruptions. And I hate people I don't know well watching me. Never liked the feel of being the center of attention.