First of all - He's a classic Midlife Crisis, thinking he doesn't have much time left and needs to go out and sleep with this younger woman to vainly try to convince himself he's not aging. Pretty classic. (And yes, they've slept together if she's sending pictures to your home.)
It may well be the best option right now to send him packing and make him live with the reality of his choices; right now he gets the comfort of his home with you while still getting to eat cake with her. And if she sent those pictures to YOU deliberately, you know already she's a bag of cats. Still, if you take this approach, you have to be willing to accept the fact that he may not come back. He may not come back regardless of what you do. But having to live in reality instead of fantasy often can wake up a MLC spouse.
Now, in order to achieve this, it sounds like you need to square up your finances. So back to my original unanswered questions: How much equity do you have in the house? If he leaves how much could you get from a roommate? Talk to an attorney to get an idea of how you would fare financially in a divorce (if he makes more than you he may owe you alimony - he might be willing to trade the house to you for that depending on the finances.)
Also - assuming those student loans were all to finance your PhD - how is your earning potential now with that PhD? Will you be able to find new work at a higher salary?
And MOST important of all - go out and get a life. Thinks of things you have always wanted to do and DO THEM. Do the things you wanted to do but your husband might have laughed at or belittled. Do the things you never dared to do. Go out and live a life so fun and exciting that your H will wonder what he's missing out on. You might find your life is so good you don;t even want him back!
When my ex left after 26 years I bought a drum kit and learned to play the drums at 53. I joined an adult rock band class and began playing in a pop punk cover band for several years. My best friend who is a professional singer songwriter began bringing me along to play glockenspiel and vibraphone at her gigs. Since my divorce 9 years ago I've met and played on the same stage with famous musicians. It's just a hobby but my exH, who thought HE was the musician in the family, must be green with envy! (He can play a dozen Neil Young songs on acoustic guitar.)
Also, although I fought hard for my marriage, EVERY man I have dated since my divorce has treated me better than my ex. NONE of them complain about the supposed flaws my ex was so focused on.
