My girlfriend has a 3yo shiba inu that's driving me crazy. I'm always stressed because I'm afraid of dogs and when I'm at her home especially because the dog always jump over the bed. So.. how can I teach the dog not to do it even if I'm not someone important in the dog's life?
This answer is to the question "how can I teach the dog"
You and the dog don't have a relationship that will support training. The first step would be mutual trust and respect, you don't have that.
Dogs are trained to either be allowed on the bed or not. In this case it appears your girl friend has all ready trained the dog to get on the bed, before you met her. Changing that training can be done, but it is difficult to unlearn an established behavior.
The easiest solution to this problem is to teach you not to be afraid of the dog. I suggest talking to your girlfriend about it and working towards a solution together. She can show you how to interact with the dog. Once you and the dog are friends, you won't be afraid.
There are bunch of related topics about dating someone with a pet, with whom you are not comfortable. The Person to Person scenarios there are out of scope here, but we can help with any person to pet issues.
You may also want to read How does the emotional bond to a pet compare to the emotional bond to a child?