It's really hard to deal with, because when I feel it, it isn't just one of those, "Oh, I can just ignore it." things. It's there and there's no way it's going away any time soon. I want to just be all by myself so I can sit and cry, but I can't do that. I have to smile and say, "I'm good!" or some form of that. I never tell them I'm just okay, it's always that I'm at least pretty good. And usually when I get this feeling, I'm at work. It doesn't seem to ever happen anywhere else and that sucks.
I'm not telling you to ignore it. I'm telling you to relax, and take time for yourself when you can. I have "sit and cry" breaks during my work. I'm not making this up. I go to the bathroom, get some of those feelings out, and then come back. You're putting a lot of negative emphasis on smiling at customers, and you don't need to be doing that. Yeah, it sucks you're faking it. But it is what it is. Try and have fun with it. Challenge yourself to be positive with guests. And then take breaks. Take breaks to feel and be 100% you. You can't change where you are while you're at work. So try really hard to put it out of your mind. Work with what you have. If you can't change the situation, make the situation as good as it can be. If it is too much for you (which it sounds like it might), perhaps you might need to find a job that you can feel more comfortable in. Customer service for sure is tough and can be hard on some people. It's all about what you can handle. I'm really really gonna advise taking those 5 minute breaks. They have made a huge difference for me throughout the summer when the aquarium where I work was crazy crowded and stressful and I was dealing with 1000s of guests every day.