I'd also like to add in Dr. Phil style that :
The best predictor of future behavior is past behavior.
Most people are who they are, and can only change from within themselves. You have been in a relationship with him for a relatively short amount of time (what? six months or so?) while his last relationship was a 4 year one, complete with an engagement. Has your guy demonstrated support for your career? No.
I'm sorry, but this guy doesn't sound like someone who will emotionally support you through this very difficult period in your life. Nursing school is tough, and even tougher when you are constantly struggling with your personal life. You need someone (if there is to be someone in your life) who will be there for you, and will 100% back you.
Only you can determine whether or not it's worth it to go through school and all the dedication it will require in a relationship with this person.
I encourage you to re-read what you posted here, and make an appropriate decision. If you decide to stay in this relationship, you guys really need to have a heart-to-heart, and he needs to put aside his past experience and understand that you and she are two different people entirely, who happen to have the same career interests.
I wonder why he has been attracted to two people in this field. Does he need "caring" for? How is HE as a person.. what does HE do for a living? What was his family life like growing up? Was his mother a nurse?
Just some food for thought.
Happy New Year to you too!