I'm so sorry to hear that this is happening to you.
When I was 17/18, I fell deeply in love with a man who was 30/31 at the time. I thought that we were special and that age was just a number. We were together for just six months before we split up, and I never stopped missing him. Four years later, when he admitted that he felt like he'd made a mistake and missed me, I couldn't say no. We gave it another try, first as partners, then as friends with benefits, but neither thing worked out. He honestly believed he was in love with me, I was close enough to him to know that he was being genuine about that... but his actions didn't match his words, I could only conclude that he was honestly confused. He just wasn't into me in the same way as before, and eventually I accepted that and let him be.
As time went on, I found out from other people who knew him well that he'd had a marked pattern of falling for younger women, and of not being able to sustain a long term relationship. He's now dating a woman four years younger than me.
Looking back, it seems likely to me that he was an ephebophile (http://en.m.wikipedia.org/wiki/Ephebophilia
). How someone like that can ever really maintain an LTR, I just don't know. :/
Anyways, it seems to me you have two choices. Accept your partner's preference and adjust to the idea that he will likely continue to collect young lovers as time goes on. Or, decide that that isn't for you, and let him go. Me, I'd opt for the latter... even if you think you could deal with it without becoming bitter, what's up with how insensitive he's been about this whole thing? He should be talking this out with you as an equal, not making demands and expecting you to suddenly uproot your life and become a gypsy (that part sounds like a plain old mid life crisis). As Dan Savage would say, DTMFA.
Best of luck.