I have just come out of a situation like that too - with my ex girlfirned (greek orthodox) for a year, travelled togehter to my home country, almost engaged... etc etc. it was very serious. 2 months later (last week), and she has now found a new boyfriend, probably as they say '' rebound ''
I know how you feel, it is hard. but we must pray. I know God allows these things which seem weird and unfair to happen to us for the best, and only in one point in our life will we know why iit had to be.
I will pray for you.
personal advice; dont look into things too much, and take one day at a time. it is all we can do!
God bless you brother.