So you were dating this guy for just a few months and now you can't figure out how to go on without him? I get that him calling it off was a shock but that's a risk you take with any new relationship. You have to figure out how to keep yourself from going so deep so fast, it's a recipe for heartbreak. Save the life-changing plans for a little later on. As for why he did it, it sounds like he was confused and a little immature.
It's worrying that you say your life seems empty when in fact you still have your spouse. Does he not fill up your life at all? If not, it doesn't sound like a very good marriage. It's also worrying that you say things seems hopeless and pointless now, when this guy was in your life for such a short time. Maybe it would help to go to therapy, to figure out how to be happy and purposeful because of yourself and your own goals in life, rather than having those things come from relationships.
Staying happy with a mate is a subject that many, many books have been written on. It's not uncommon to find it waning over time, and if you want love all you can do is try to be very very choosy in relationships, build them slowly, then invest in making them work. But first you have to be happy with yourself, and it doesn't sound like you are.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.