In your case, you really need to make her trust you. Trust is not something that can be earned on a single day. You have to give her time to gain trust. Meanwhile, you can try not to do anything that can lead you to be suspicious. If possible take her to your work so that she can see that you are not doing anything wrong. Introduce her as your girlfriend to your customers, this might give her assurance that you are not cheating. Make her understand that you are only trying to do your job and nothing else. You can also try to make some time out of your work and spend some time with her. This will make her feel special and she will start to trust you more. She might also feel ignored, so give her time and attention too.