hello you have been very brief in your question so makes it a bit more complicated to answer.

did you both argue?
was he married or in another relationship?

there are loads of questions one could ask but if the answer to first two were no then i feel he just wanted to move on.
doctors are very on demand and maybe he felt he could not combine late hours or early starts in a relationship.
im not making excuses for him as 3 months is along time but only mention sex and him breaking up with you . i shall be willing to advise you further.
im always here to give support as you abviously distressed and upset about this and my advise to you is move its his loss.
if he thought so much of you he would never have left. feel free to post again we are here to support you.
don,t put your life on hold because he has moved on. get out there and enjoy yourself.

in my early years i've dated a register nurse and he would see me for 6 months, then duck me for 3 months, then call me and i would see him again for 6 months. okay you get the picture. you have to know the guys intentions. i knew the guy met someone else. just that simple!
some guys loose interest! do you expect him to tell you that?
i was talking to another guy a few years ago, and he stop calling everyday and would call once in a blue mood to check on me. i was confused, but thinking about it, he just wasnt that into me.
guys are jerks!