High five for being a great mom! It is not often that 15 year old daughters are described as the best and great. Life will change that is for sure however I think empty-nest is a fraud. The world that our kids are growing up into is more difficult than the world that we grew up into. My kids are 25 and 27 and living at home...again. They are both transitioning into their next lives. The fact that you and your daughter are still close at this point really shows that she will be close no matter what else is going on in her life.
You have time on your side right now. Dig out that list of all the things that you have put on the back burner for "someday" and brush it off. Start making a plan now so that by the time you wonderful daughter goes to college you have a way of going with the flow. My dad was looking retirement in the face after an extremely busy and committed career in emergency medicine. Years before he retired he got certified as a technical delegate for dressage horse shows (it is the rules keeper). Now that he is retired he has a second career working at the shows and just loves it.
This could even be time that you could look at some sort of career change so that physical limitations are not an issue. Use this time while she is still home to plan the next phase of your life. Not only will you be better off when DD goes to school but you will also be modeling really great things for your DD . I'd put money on the idea that she will be very proud of you.
Keep in touch.