I would like to echo many of the sentiments here. My wife knew that medical school was my goal when we met, so I didn't have to deal with starting the conversation out of the blue. She was and is extremely supportive, and really pushed me to get into med school and to do my best once I was in. She is not in medicine, but is a professional herself, so she understands some of the commitment, which helps. I imagine any individual committed to performing well in their career, whether professional or not, will have a similar thoughts about it. My wife has taken care of me marvelously, and has helped to keep me in my study hole at times when all I wanted was to sit on the couch with her. Her help with things like chores, meals, vacation planning, has also been invaluable.
But that does not mean it has been all flowers and potpourri. "Crises" happen to everyone at one time or another, and they get magnified for people (like medical students) who don't have as much time to deal with them. Adding the second person just complicates it. If your significant other needs support in the few days before a test, you have to give them some. That could cost you some points, and both of you need to understand that, and be ok with it. There are plenty of days where I have wanted to just shut down at the end of a long day of studying. But for all the support my wife has given me, she has earned more quality time with me than she is getting. So I make it a point that every day she gets time with me. It might seem like it's a huge drain on you at first, but over time, you'll realize that it's just as refreshing to you as it is to your significant other.
In the end, just keep communicating. Don't make big decisions without your significant other. Keep them in the loop about what you are feeling as far as specialty, location, etc. Don't expect them to follow you blindly all over the country. Yes, you will probably have to apply to schools / residencies in locations they don't want to go to. Spend the time to make sure they understand why, and make some compromises about things you don't care as much about. The more truthful information you can give them, the better.