2004 BMW automatic transmissions problem

I have a 2004 bmw 525i and the car will not go on reverse anymore. It still drives but does not change gears. Drives only on first gear. I have taken the car to different mechanics and most say that I will need a rebuild transmission or a new one. However, some other mechanics have mentioned that it could be a pressure sensor as the reverse requires more pressure or force to be used. I just want to know if anyone has experienced this problem because I do not want to spend so much on replacing the transimission if its just a sensor. Thank you