Arians don't often do so well in a medical field. Truth is there is a alot of faceless, selfless sacrifice involved in a career medicine and Aries wants action and recognition, not to slave away in med schools, clinics, libraries, labs and board meetings. This is why Virgos are ideal for medicine, but not Arians.
Witha Pisces NN, that's teh direction you ought to be moving in. Pisces is spiritual, empathetic and universal. there's a selflessness about Pisces that will be difficult for your Arian persona to acchieve, but it should serve as a guide for you. While you do need to seek out the affirmations and respect from others that your Arian energies need, you should temper your Arian drives with Piscean gentleness and selflessness.
The thing about Pisces, and especially with Pisces in the 12th house, is that it's not obvious. Neptunian energies are always below the surface, swimming around in a sea of emotion, intuition, mystery and spirit. While Aries wants things on the surface, visible, and tangible, it's clear that with your NN in Pisces and in the 12th, much of your life path will be shrouded and mysterious. You need to cultivate faith and be ready to walk on even when your path is unclear, trusting your intuition and the mysteries of the universe to guide you. As an Aries you'll be tempted to rely solely on your vision, will and determination, but you'll need to recognize that there will be times in your life that those are not the right tools to be using, but instead the universe has something very different in mind for you. Aries is a fighter, but it may be the most important lesson you need to learn in this life is not to fight like an Arian, but to surrender like a Piscean to forces greater and wiser than yourself.