I know first hand how hard it is to fit exercise into a busy schedule and the only advice I can offer is you just have to put it in your schedule somewhere. Make time for you and it will pay off in the end! I was always trying to "fit" exercise into my day, but once I said this is what it is (M-W-F 4:30-4:45) is my time than it got easier. It can cause conflict at times, but I just let everyone in my life know that I have to do this for me. Even if you can only fit it 20-30 minutes in at a time at least it is something. I am sure being in medical school is intense and I wish you the best in finding something that works for you.