Why did he promise to marry me if he was just going to leave?

I think the best person to answer that question is him, and only him. But you need to know, hun, that promises are merely words and they have this uncertainty among them. It's still best not to hold on to words as much as one does to actions.

People often plan to do one thing, and then do another thing. They often have the best intentions to help you, but then their own limitations come up and kinda take over. Ultimately, it's anyway your job to make you happy, so take support from others, but depend on yourself and the divine -- who will always be there for you. It is very heartbreaking though, so my heart goes out to you. Companionship can be one of the sweeter things we experience, and to have it taken from you as it was promised can be very painful -- but you'll get through it -- I promise.

Something must have changed inside of him. Perhaps he did genuinely believe he wanted to marry you one day, but sometimes happiness does not have a straight cause and we have to keep trying and keep believing that one day it will come.

Things may have changed in his mind... that's no reason to do it of course- but the human mind is complex and complicated- maybe something happened to change his mind.