Jealousy happens. It happens to everybody. But the girl you are with should be so into you that it hurts you like a brick in the face. If you've pulled the right moves, you shouldn't even bother being jealous, she adores you. If you haven't snagged a girl like that, then there is a problem.
The truth is girls do weird things. They might complain about guys honking and whistling at them in the street, but deep down they enjoy the attention. They might complain about the guys who keep hitting on them, but they enjoy it enough to never do anything about it or tell them that it makes them uncomfortable unless the guy is totally repulsive or making far too many moves. It's just the way many girls are, whether they are dating someone or not. If you have a devoted girlfriend, she'll tell them to step off, but many girls just don't.
First, quit caring so much about her. Start caring about you. You have work to do. If you're chroming hydraulic cylinders, keep your mind on that. If she didn't like you, she wouldn't be with you, unless there was some offset reason such as money, attention, whatever. Let her do whatever she wants during the week and quit worrying about it. You can tell if your girlfriend isn't really happy with you, which at that point, you should just break it off because it only goes downhill from there.
And as the guys say, definitely don't show it. Just as a man, you learn to hide pain and emotion. Be a man and don't show your jealousy. You're better than any two-bit carbon copy guy out there, so think that way. When you do that, she'll naturally be attracted to you. You're just better than everyone else and you act like it. If you're doing things right, you have nothing to worry about, so don't worry and keep your mind set on yourself.