I don't know what to do? Please help!!?

I currently have a self trained service dog who has been a huge help to me for the last few years. He has recently developed a slipped disk in his back which the vet says may always cause him problems and that might be best for him to retire and just be an inside dog. Over the last few months I can tell that he is struggling when we go out but he loves going with me anyways. I hate to see him in pain and he can't do his job as well so I think the vet may be right that he just needs to retire to be a pampered pet. Also the pain medication that he is on makes him drowsy and kind of grumpy so he doesn't do his job as well because of that. It will be very difficult for me to go back to life without a service dog. I would like to find and start training a new dog but the only problem is convincing my mom. I can be completely financially responsible for both my dog now and a new one and I can care for them both. I am an adult and I have a job but I just need to live in the same house with my mom because of my disability. She just doesn't want another dog in the house but we have plenty of room. All she would need to do is agree to let the dog live in our house. I can do the rest and I have trained dogs my whole life so behavior won't be a problem. Since I have had a service dog I have been able to be much more independent at home and out in public but for some reason my mom doesn't see how much a service dog helps me. She said I could get a new service dog when my current dog dies but that will hopefully a long time from now because he is still young and there is no way I would ever give him to another home because he has been my constant partner and friend for years. But since he can't do his job well anymore I need to start training a new dog since it takes quite a while to train one. How can I convince her to let me find another dog to start training? I was considering just starting to look for one but if I find one, I don't want her to be mad at me and not let me keep the dog. The only other thing I could do would be to move out but I really need the support of my family. Please help?