I know it's hard to help who you fall for, but girl - he's married :(
The age thing is not the issue here, it's the fact that he's already hitched and you're into him. It's really not the best place to be.
I'm only going to suggest the harsh option, but probably the best one: walk away from the situation.
Talk to him and tell him amicably that you are leaving him. It's a firing shot so you both know where you are heading. Don't ask but tell him you expect his compliance in that he will not contact you. Delete his number/email/address etc that you have and don't google him. When you start your PhD I suggest that you throw yourself into your studies. If he pops into your mind, do something that will create a positive diversion such as going for a walk or listening to upbeat music (in my experience, listening to sad love songs and looking at old photos is the worst thing you can do). Don't jump into a rebound relationship, ride out your feelings but make sure you keep on socialising.
I know this is a step-by-step, but it has worked for me when I've fallen hard for people. I always do the 'cold turkey' dropping contact because I move on faster that way; If I call the person, the rose tinted glasses go back on and I want them back like crazy. Honestly, just break away. You will have to have willpower but you can do it if you know deep down it is for the best.